The effect of phencyclidine (PCP) on the release and synthesis of [3H]-dopamine ( [3H]-DA) newly synthesized from [3,5-3H]-L-tyrosine was studied under both basal and potassium-stimulated conditions in slices from the rat striatum. Phencyclidine (3-100 microM) stimulated the release of [3H]-DA during superfusion with low level potassium (4.5 mM) buffer, but had no effect on the release of [3H]-DA elicited by superfusion with buffer containing 40 mM potassium. On the other hand, phencyclidine had no effect (except at 100 microm) on the spontaneous release of [3H]-H2O (formed as a result of the hydroxylation of [3,5-3H]-L-tyrosine), but inhibited the increase in formation of [3H]-H2O normally associated with potassium-induced depolarization. These data are discussed in relation to results obtained with phencyclidine on other in vitro and in vivo models of dopaminergic function.