Reserpine administration (5 mg/kg, i.p.) to guinea pigs resulted in marked and long lasting dopamine (DA) depletion and a rapid, short lasting, increase of 3,4-dihydroxyphenylacetic acid (DOPAC) in the striatum. A marked and sustained increase of the level of 5-S-cysteinyl-dopamine, which is an adduct presumably formed following autoxidation of DA, started 3-4 h following the DA and DOPAC changes. Only small changes in the levels of the 5-S-cysteinyl adducts of DOPAC and 3,4-dihydroxyphenylalanine (DOPA) were found.