diff --git a/cara/models.py b/cara/models.py
index dd8262bb..69f642a6 100644
--- a/cara/models.py
+++ b/cara/models.py
@@ -287,9 +287,8 @@ class Model:
         elif t0 < t <= t1:
             # Concentration while infected present.
             init_concentration = self.concentration(t0)
-            time_present = t - t0
-            fac = np.exp(-IVRR * time_present)
-            return ((ER + Ni) / (IVRR * V)) * (1 - fac) + init_concentration * fac
+            fac = np.exp(IVRR * (t0 - t))
+            return ((ER * Ni) / (IVRR * V)) * (1 - fac) + init_concentration * fac
         else:
             # Concentration while infected not present.
             end_concentration = self.concentration(t1)
diff --git a/cara/tests/test_known_quantities.py b/cara/tests/test_known_quantities.py
index 8976a558..b87a318b 100644
--- a/cara/tests/test_known_quantities.py
+++ b/cara/tests/test_known_quantities.py
@@ -64,7 +64,7 @@ def test_r0(baseline_model):
     concentrations = [baseline_model.concentration(t) for t in ts]
     npt.assert_allclose(
         concentrations,
-        [0.000000e+00, 2.909211e-01, 1.273836e-04, 2.909210e-01, 5.577662e-08],
+        [0.000000e+00, 2.891970e-01, 1.266287e-04, 2.891969e-01, 5.544607e-08],
         rtol=1e-5
     )