The effects of 1-20 mg/kg diazepam were studied on the extracellular concentrations of dopamine (DA), dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the nucleus accumbens and striatum of conscious rats, using intracerebral microdialysis. Five, but not 1 mg/kg diazepam significantly reduced extracellular DA, DOPAC and HVA in the nucleus accumbens. Twenty mg/kg diazepam significantly reduced extracellular DA, DOPAC and HVA in the striatum. A significant effect on striatal DOPAC, but not on DA and HVA, was seen with 10 mg/kg diazepam, while no changes were found with 5 mg/kg diazepam. The results suggest that diazepam reduces the release and metabolism of DA in the nucleus accumbens more than in the striatum.