Chronic lithium (Li) administration to rats was found to inhibit the hyperactivity induced by 0.5 mg/kg d-amphetamine but not that induced by 1.0 mg/kg d-amphetamine. Contradictory results on the ability of Li to inhibit amphetamine-induced hyperactivity may be due to the different dosages of amphetamine employed. 2 days after withdrawal from 3 weeks of chronic Li administration, rats showed no rebound supersensitivity of the hyperactivity response to 0.5 mg/kg d-amphetamine. Surprisingly, Li-withdrawn rats showed a marked subsensitivity of the stereotypy response to apomorphine. Such subsensitivity to apomorphine in Li withdrawal may relate to recent reports of homovanillic acid increases in rat brain in Li withdrawal, and to clinical reports of Li withdrawal inducing psychosis.