The purpose of the present study was to characterize the metabolism of dopamine (DA) in tuberoinfundibular (TI) neurons terminating in the median eminence and to examine the effects of procedures that alter the synthesis and turnover of DA in these neurons on the concentrations of dihydroxyphenylacetic acid (DOPAC) in the median eminence. The DA uptake inhibitor nomifensine (25 mg/kg, i.p.; 30 min) failed to alter median eminence DOPAC concentrations indicating that very little released DA is recaptured and metabolized by TIDA neurons. Within 5 min following the administration of the monoamine oxidase inhibitor pargyline (50 mg/kg, i.v.) median eminence DOPAC concentrations declined to 15% of control demonstrating that this metabolite has a high turnover rate and is rapidly removed from the median eminence. Median eminence DOPAC concentrations in diestrous female rats, whose TIDA neuronal activity is higher than in the male, were two-fold greater than in male rats. Prolactin (10 micrograms/rat, i.c.v.; 12 h), which increases TIDA neuronal activity, produced a corresponding increase in median eminence DOPAC concentrations in male rats. Restraint stress (30 min), which decreases TIDA neuronal activity, produced a corresponding decrease in median eminence DOPAC concentrations in diestrous female rats. The results from the present study suggest that DOPAC concentrations in the median eminence can be used as an index of TIDA neuronal activity.