Serum samples were selected randomly from blood samples drawn in the morning for other reasons in patients treated with citalopram, and the serum concentrations of citalopram (S-citalopram) and its major metabolite desmethylcitalopram were determined. A total of 44 patients, 13 males and 31 females, with a median age of 38.5 years (range 18-89) entered the study. In 17 patients given 20 mg/day of citalopram the S-citalopram was (median and range) 153 nmol/l (83-237). In 24 patients treated with citalopram 40 mg/day the S-citalopram was 240 nmol/l (range 0-360). In one serum sample no S-citalopram could be detected (non-compliance) and in another sample S-citalopram was just above the detection limit (about 3 nmol/l). The latter may at least partly be due to treatment with 600 mg/day of carbamazepine, which is known to accelerate the metabolism of other drugs. The serum concentration of the major metabolite desmethylcitalopram averaged 28% of S-citalopram and is most likely without clinical importance. It is concluded that determination of S-citalopram may be considered if there is doubt about the compliance of the patient, in drug interaction cases or if the clinical effect is poor. If high S-citalopram and troublesome side-effects occur simultaneously the serum level may be lowered by dose adjustment.