Concentrations of clomipramine, a specific and potent serotonin uptake inhibitor, are measured in 67 psychiatric patients and 12 normal volunteers. The psychiatric patients are grouped according to the DSM III R criteria namely; pathological gamblers, obsessive compulsives and sufferers of panic disorders. Before and 30, 60, 90 and 120 min after an intravenous infusion of the drug (12.5 mg in 10 min), serum samples are collected to evaluate the concentrations of cortisol, prolactine and growth hormone. Simultaneously the clomipramine concentration of these samples is determined and these results only are reported in this communication. Very different drug concentrations are observed in individual patients receiving the same amount of drug, indicating a substantial inter-individual variability of drug metabolism. No statistical differences (Newman-Keules test) between the clomipramine concentrations from the patients of the three psychiatric groups and the normal group are observed. Neither are statistical correlations observed when clomipramine concentrations from all individuals (n = 79) are related with the age, sex or consumer behaviour (cigarette smoking, alcohol and coffee intakes) of the patients.