The i.v. pentylenetetrazol seizure threshold was increased by 2.5 mg/kg and decreased by 15 mg/kg of a single (+)-amphetamine dose. After 7 consecutive days of amphetamine administration, tolerance developed to the decrease but not to the increase in seizure threshold. At 12--48 h after the last dose of 2.5 mg/kg seizure threshold was decreased, and at 36--48 h after the last dose of 15 mg/kg seizure threshold was increased. After acute and chronic administration of (+)-amphetamine (2.5 mg/kg) endogenous concentrations of whole brain dopamine (DA) were increased and returned to normal levels during the withdrawal period (12--48 h); endogenous norepinephrine (NE) levels were unchanged by acute and chronic drug treatment and during withdrawal. The rates of DA and NE synthesis were increased by chronic amphetamine administration at 24--48 h after drug withdrawal. An acute dose of (+)-amphetamine (15 mg/kg) decreased endogenous levels of DA and NE; normal levels of DA were detected with chronic drug treatment and during withdrawal, with NE remaining slightly depressed. The rates of synthesis of DA and NE were increased by acute and chronic amphetamine treatment and returned to normal 24--48 h after withdrawal. The rebound reversal in seizure threshold after (+)-amphetamine withdrawal suggests an abstinence syndrome that may be interpreted as evidence for the development of physical dependence to (+)-amphetamine after chronic drug administration.