||The transcerebral dialysis method has been utilized for measuring extracellular brain concentrations of serotonin and 5-hydroxyindolacetic acid. Dialysis fibres were implanted transversally in the rat frontal cortex and perfused by Ringer. Serotonin and 5-hydroxyindolacetic acid were quantified by reverse phase high performance liquid chromatography with electrochemical detection. Experiments were performed in freely-moving rats 20-24 h after the implant of the fibre. Basal output of serotonin and 5-hydroxyindolacetic acid was 0.12 and 22.8 pmol in 20 min, respectively. The output of serotonin was calcium-dependent and tetrodotoxin-sensitive (1 micron in the Ringer) while was stimulated by veratridine (50 microns) and by high concentrations of K+ (60 and 100 mM). Serotonin output was increased in a concentration-dependent manner by chlorimipramine (1-10 microM) in the Ringer; this drug stimulated serotonin release also when administered s.c. (20 mg/kg) in a tetrodotoxin-sensitive manner. The irreversible monoamine-oxidase inhibitor pargyline (75 mg/kg, i.p.) strongly stimulated serotonin output while reduced 5-hydroxyindolacetic acid output. A proposed serotonin releaser, fenfluramine (25 mg/kg, s.c.), stimulated serotonin release and this effect was strongly potentiated by local application of tetrodotoxin (1 microM). Agonists of serotonin receptors such as lisuride (0.03 mg/kg, s.c.), 8-hydroxy-2-(di-n-propilamino)tetraline (0.25 mg/kg, s.c.) and 5-methoxy 3(1,2,3,6-tetrahydro-4-pyridinil)-1H indole succinate (1 mg/kg, s.c.) reduced serotonin release. It appears that brain dialysis is a suitable method for the study of serotonin release in the cortex of freely-moving rats.