Serotonin turnover was studied in discrete brain areas of rats treated with various antidepressants. Changes in turnover were estimated using the probenecid, pargyline and decarboxylase inhibitor methods. Chlorimipramine decreased serotonin turnover in the hypothalamus, hippocampus, cortex, septum and nucleus caudatus in doses as low as 1.8 mg/kg, a dose similar to that used in man. This effect on turnover was seen after another drug (fluoxetine) that affects 5HT reuptake, but not after antidepressants that do not affect 5HT reuptake (desmethylimipramine, amitryptiline, iprindole or amphetamine).