diff --git a/bluecellulab/cell/injector.py b/bluecellulab/cell/injector.py
index 6b08100d..8b832e63 100644
--- a/bluecellulab/cell/injector.py
+++ b/bluecellulab/cell/injector.py
@@ -243,8 +243,14 @@ def add_replay_noise(
         """Add a replay noise stimulus."""
         if section is None:
             section = self.soma  # type: ignore
-        mean = (stimulus.mean_percent * self.threshold) / 100.0  # type: ignore
-        variance = (stimulus.variance * self.threshold) / 100.0  # type: ignore
+
+        if stimulus.mean is not None:
+            mean = stimulus.mean
+            variance = stimulus.variance
+        else:
+            mean = (stimulus.mean_percent * self.threshold) / 100.0  # type: ignore
+            variance = (stimulus.variance * self.threshold) / 100.0  # type: ignore
+
         tstim = self.add_noise_step(
             section,  # type: ignore
             segx,
diff --git a/bluecellulab/stimulus/circuit_stimulus_definitions.py b/bluecellulab/stimulus/circuit_stimulus_definitions.py
index fae177f1..2849c8bb 100644
--- a/bluecellulab/stimulus/circuit_stimulus_definitions.py
+++ b/bluecellulab/stimulus/circuit_stimulus_definitions.py
@@ -242,11 +242,17 @@ def from_sonata(cls, stimulus_entry: dict, config_dir: Optional[str] = None) ->
             raise ValueError("Stimulus entry must contain either 'node_set' or 'compartment_set'.")
 
         if pattern == Pattern.NOISE:
+            has_mean = "mean" in stimulus_entry
+            has_mean_percent = "mean_percent" in stimulus_entry
+            if has_mean == has_mean_percent:
+                raise ValueError("Noise input must contain exactly one of 'mean' or 'mean_percent'.")
+
             return Noise(
                 target=target_name,
                 delay=stimulus_entry["delay"],
                 duration=stimulus_entry["duration"],
-                mean_percent=stimulus_entry["mean_percent"],
+                mean=stimulus_entry.get("mean"),
+                mean_percent=stimulus_entry.get("mean_percent"),
                 variance=stimulus_entry["variance"],
                 node_set=node_set,
                 compartment_set=compartment_set,
@@ -380,8 +386,16 @@ def from_sonata(cls, stimulus_entry: dict, config_dir: Optional[str] = None) ->
 
 @dataclass(frozen=True, config=dict(extra="forbid"))
 class Noise(Stimulus):
-    mean_percent: float
     variance: float
+    mean: Optional[float] = None          # nA
+    mean_percent: Optional[float] = None  # % of threshold
+
+    def __post_init__(self):
+        # exactly one of mean / mean_percent must be provided
+        if (self.mean is None) == (self.mean_percent is None):
+            raise ValueError("Noise stimulus must define exactly one of 'mean' or 'mean_percent'.")
+        if self.variance < 0:
+            raise ValueError("'variance' must be >= 0.")
 
 
 @dataclass(frozen=True, config=dict(extra="forbid"))
diff --git a/tests/test_circuit/test_simulation_config.py b/tests/test_circuit/test_simulation_config.py
index 6fb568c9..fc250c50 100644
--- a/tests/test_circuit/test_simulation_config.py
+++ b/tests/test_circuit/test_simulation_config.py
@@ -73,7 +73,7 @@ def test_init_with_invalid_type():
 
 def test_get_all_stimuli_entries():
     sim = SonataSimulationConfig(multi_input_conf_path)
-    noise_stim = Noise("Mosaic_A", 10.0, 20.0, 200.0, 0.001, node_set="Mosaic_A")
+    noise_stim = Noise(target="Mosaic_A", delay=10.0, duration=20.0, mean_percent=200.0, variance=0.001, node_set="Mosaic_A",)
     hyper_stim = Hyperpolarizing("Mosaic_A", 0.0, 50.0, node_set="Mosaic_A")
     pulse_stim = Pulse("Mosaic_A", 10.0, 20.0, 0.1, 25, 10, node_set="Mosaic_A")
     linear_stim = Linear("Mosaic_A", 10.0, 20.0, 0.1, 0.4, node_set="Mosaic_A")
diff --git a/tests/test_stimulus/test_circuit_stimulus_definitions.py b/tests/test_stimulus/test_circuit_stimulus_definitions.py
index 4fd74986..dd8bf74e 100644
--- a/tests/test_stimulus/test_circuit_stimulus_definitions.py
+++ b/tests/test_stimulus/test_circuit_stimulus_definitions.py
@@ -14,7 +14,7 @@
 # limitations under the License.
 
 import pytest
-from bluecellulab.stimulus.circuit_stimulus_definitions import Pattern
+from bluecellulab.stimulus.circuit_stimulus_definitions import Noise, Pattern, Stimulus
 
 
 def test_pattern_from_sonata_valid():
@@ -40,3 +40,35 @@ def test_pattern_from_sonata_invalid():
     """Test that an invalid pattern raises a ValueError."""
     with pytest.raises(ValueError, match="Unknown pattern unknown_pattern"):
         Pattern.from_sonata("unknown_pattern")
+
+
+def test_noise_requires_exactly_one_mean_field():
+    """Noise dataclass should validate exactly one of mean/mean_percent."""
+    with pytest.raises(ValueError, match="Noise stimulus must define exactly one of 'mean' or 'mean_percent'."):
+        Noise(target="T", delay=0.0, duration=1.0, variance=0.1)
+
+    with pytest.raises(ValueError, match="Noise stimulus must define exactly one of 'mean' or 'mean_percent'."):
+        Noise(target="T", delay=0.0, duration=1.0, variance=0.1, mean=0.01, mean_percent=5.0)
+
+
+def test_noise_negative_variance_raises():
+    """Noise variance must be non-negative."""
+    with pytest.raises(ValueError, match="'variance' must be >= 0."):
+        Noise(target="T", delay=0.0, duration=1.0, variance=-0.1, mean=0.01)
+
+
+def test_from_sonata_noise_requires_one_mean_field():
+    """Parsing SONATA noise stimulus enforces exactly one mean field."""
+    base = {
+        "module": "noise",
+        "delay": 0.0,
+        "duration": 1.0,
+        "variance": 0.1,
+        "node_set": "T",
+    }
+
+    with pytest.raises(ValueError, match="Noise input must contain exactly one of 'mean' or 'mean_percent'."):
+        Stimulus.from_sonata(dict(base))
+
+    with pytest.raises(ValueError, match="Noise input must contain exactly one of 'mean' or 'mean_percent'."):
+        Stimulus.from_sonata({**base, "mean": 0.01, "mean_percent": 5.0})