Summary:Plasma nor1‐chlorpromazine (nor1‐CPZ) and chlorpromazine concentrations were measured by gas chromatography—mass fragmentometry in 15 inpatients receiving stable doses of chlorpromazine. Nor1‐CPZ concentrations averaged 20% of chlorpromazine concentrations and were highly correlated with them. Relative concentrations of nor1‐CPZ increased with age. Nor1‐CPZ concentrations per milligram dose were higher in black than in white patients.