The antidepressant drug clomipramine (CIM) blocks 5-hydroxytryptamine (5-HT) uptake in vitro. Electrophysiological studies have shown that CIM also reduces the firing of serotonergic neurons in the dorsal raphe nucleus. In order to assess the effects of CIM on serotonergic transmission in vivo, the technique of intracerebral microdialysis was used. CIM was administered either through the dialysis probe or i.p., and dialysate 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) were determined in frontal cortex and/or raphe nuclei. In addition, the action of extracellular 5-HT in raphe nuclei on the release of 5-HT in frontal cortex was studied. The administration of CIM through the dialysis probe increased dialysate 5-HT in frontal cortex in a dose-dependent fashion. An actual ED50 of 3.15 microM CIM for the in vivo inhibition of 5-HT uptake can be calculated in this brain area. When given systemically (10 or 20 mg/kg i.p.), CIM did not increase dialysate 5-HT in the frontal cortex. The occurrence of extracellular 5-HT in the the raphe area was demonstrated. This pool of 5-HT increased markedly after local (10 or 40 microM) or systemic (20 mg/kg i.p.) administration of CIM. We also examined the effect of CIM applied locally in the raphe nuclei on extracellular 5-HT in the frontal cortex. The increased dialysate 5-HT in raphe after 10 or 40 microM CIM paralleled a decrease of dialysate 5-HT in the two areas correlated negatively. The administration of CIM through the dialysis probe slightly decreased dialysate 5-HIAA in the frontal cortex.(ABSTRACT TRUNCATED AT 250 WORDS)