The stereotyped behavioral syndrome induced in the rat by apomorphine was enhanced by acute systemic administration of PCPA. This effect was dependent on the dose of PCPA and half-maximal at approximately 150 mg/kg, i.p.; it occurred within 30 min, was greatest between 1 and 5 h and had nearly disappeared by 24 h after an acute dose of PCPA. A similar effect was not found at 24 or 48 h following 3 repeated doses of PCPA of 300 mg/kg/day. This effect of PCPA was not reversed by 5-HTP or by high doses of a decarboxylase inhibitor. PCPA alone did not produce stereotyped behavior, although it produced some behavioral excitation in high doses following inhibition of monoamine oxidase. This acute behavioral effect of PCPA to potentiate apomorphine-induced stereotyped responses is unexplained. It does not seem to be due to depletion of 5-HT or to the formation of an amine as an active metabolite. We suggest that PCPA can have behavioral excitatory actions independent of its 5-HT-depleting action.