Orthostatic hypotension, the clinically most important side effect in treatment with tricyclic antidepressants, was investigated in a double-blind study with clomipramine and the selective serotonin reuptake inhibitor citalopram given for 5 weeks. All patients were initially given placebo for 1 week. In the clomipramine group (n = 17) a significant orthostatic drop in the systolic blood pressure was observed during treatment; this remained significant over the whole investigational period. A curvilinear correlation was demonstrated between the orthostatic drop in systolic blood pressure and the plasma levels of clomipramine and desmethylclomipramine. The most pronounced orthostatic reaction was thus seen in 1-2 weeks, at plasma levels of 25-75 micrograms/l (clomipramine). The correlation between the subjective symptoms and the measured orthostatic drop was poor, as was the correlation between the subjective symptoms and the plasma levels of the two active compounds. The change in orthostatic heart rate during clomipramine treatment was insignificant. In the citalopram group (n = 15) no significant changes in orthostatic blood pressure or heart rate were demonstrated during treatment and these patients had no orthostatic complaints.