The paper analyzes the impact of a vehicle pollution charge (Ecopass), enforced at peak time, on nitrogen oxides concentration in Milan. Using hourly data on pollution concentration and a vector auto regressive model, I estimate the short and long run effects of the policy, the effects outside the Ecopass area and during off-peak time. Results suggest that Ecopass reduced pollution in the short run, but had no effect in the long run. The effect on zones outside Ecopass area is not homogeneous, suggesting substitution effects in some areas of the city. There is no systematic evidence of increased pollution levels during off-peak time due to Ecopass.