The effects of the alpha2-adrenoceptor drugs, medetomidine and atipamezole, on dopamine overflow evoked by low (6 Hz-10 s) and high (50 Hz-4 s) frequency electrical stimulation of the median forebrain bundle were studied in striatum of BALB/C mice anaesthetized with chloral hydrate with fast in vivo voltammetry techniques. The effects of these drugs on the basal concentrations of dopamine metabolites were also investigated by means of differential pulse voltammetry. Medetomidine dose dependently decreased dopamine overflow in nucleus accumbens in the dose range 5-100 microg/kg, s.c. This effect was seen only at low frequency stimulation and reached 85% at a dose of 100 microg/kg. Medetomidine also decreased the basal concentration of striatal homovanillic acid. This effect did not exceed 35%. Atipamezole antagonized the inhibitory effects of medetomidine on the dopamine overflow. but showed no effect itself. We suggest that alpha2-adrenoceptors in dopaminergic terminal fields in the mouse striatum are involved in the regulation of dopamine release at physiological stimulation frequencies.