Previous studies using radioimmunoassay procedures have failed to show age-related changes in the concentration of Met-and Leu-enkephalin or neurotensin in rat basal ganglia. In contrast, using a combined high-pressure liquid chromatography (HLPC)- radioimmunoassay (RIA) technique we now report considerable decreases in the levels of these neuropeptides in areas of basal ganglia of 22 months-old compared to 3 months-old male Wistar rats. The concentration of Met-enkephalin was greatly reduced in the striatum and nucleus accumbens, but not in substantia nigra, of old compared to young animals. There was a similarly large decrease in Leu-enkephalin content in striatum of old rats with less marked decreases occurring in both the nucleus accumbens and substantia nigra. Neurotensin levels in the striatum and substantia nigra were greatly reduced in old rats, with a less marked decrease in the nucleus accumbens.