Cocaine and its metabolites were measured in plasma and brain from mice injected i.p. with cocaine and monitored for spontaneous locomotor behavior. Cocaine concentrations in the brain reached peak values within 5 min after administration of cocaine. At all time points between 5 and 60 min the concentrations of cocaine in the brain were 7-fold higher, on the average, than those in plasma. The opposite was true for the concentrations of benzoylecgonine; brain to plasma ratios of benzoylecgonine were approximately 0.1 from 5 to 30 min after i.p. cocaine injection. After i.p. injection of either 10 or 25 mg/kg of cocaine, cocaine disappeared from plasma and brain with a half-life of 16 min and benzoylecgonine disappeared from plasma with a half-life of 62 min. There was good correspondence between locomotor stimulation and concentration of cocaine in the brain measured at 12, 22 and 32 min after i.p. administration of 25 mg/kg of cocaine. Among individual animals there was a significant correlation between their locomotor stimulation and their brain cocaine concentration, indicating that differences in cocaine levels in the brain between animals contribute to their different behavioral response; however, the correlation analysis also indicated the role of other factors determining the locomotor response to cocaine.