Five healthy volunteers took part in a crossover study which examined the serum concentrations of dothiepin and northiaden after a 25 mg three times a day and a 75 mg once a day dosage regimen of Prothiaden. The inter-individual variation of serum levels was large after either schedule which is to be expected with this group of drugs. The minimum steady-state level of dothiepin tended to be lower after the single daily dose, but the differences were small and not statistically significant. The approximate maximum steady-state levels of dothiepin showed large intra-and inter-subject variation and no obvious trend. The values of the desmethylated metabolite, norhiaden, tended to follow the dothiepin concentrations but were lower than the parent drug. Average steady-state levels tended, with one exception, to be very similar after both regimens with no evidence with no evidence fo any trend when comparing the two regiments. The study showed that the two regimens yielded similar steady-state serum concentrations both of drug and metabolite but inter-individual differences were large.