Methotrimeprazine (levomepromazine) has two major metabolites in man: N-monodesmethyl methotrimeprazine, which is pharmacologically active and almost as potent as the parent drug; and methotrimeprazine sulfoxide, which is much less active. Blood levels and the distribution between plasma and erythrocytes of methotrimeprazine and the two metabolites were studied in five psychiatric patients on oral methotrimeprazine and after incubation of the compounds in blood from healthy volunteers. The concentrations were measured separately in plasma and erythrocytes by gas chromatography with a nitrogen detector, and the concentrations in whole blood were calculated from the plasma and erythrocyte concentrations. In four of the five patients the blood levels of both metabolites were similar to or higher than the levels of the parent drug. A large interindividual variation was observed in the plasma-erythrocyte concentration ratios. The mean ratios in all individuals were 1.76, 0.57, and 3.02 for methotrimeprazine, N-monodesmethyl methotrimeprazine, and methotrimeprazine sulfoxide, respectively. The relatively high blood concentrations of N-monodesmethyl methotrimeprazine suggest that this metabolite may contribute significantly to the therapeutic action and side effects of oral treatment with methotrimeprazine.