The time-course of apomorphine in brain was studied in 7-day-old and young adult rats following i.p. injection of 10 mg/kg apomorphine. In the adult animals continuous stereotypes behaviour (SB) commenced within 1-2 min and was no longer present at 75 min. The peak concentration of apomorphine was present at 5 min and then declined exponentially (half-life = 10.5 min); only trace amounts (less than 0.01 mug/g) were present at 90 min. In the immature rat apomorphine induced an initial phase of intermittent locomotion lasting about 10 min after which the rats appeared sedated; 60-80 min after injection intermittent SB emerged and terminated 150-180 min after the apomorphine injection. The peak brain concentration of apomorphine was present at 10 min and then declined exponentially (half-life = 28 min). Significant amounts of apomorphine were still present in brain at 150 min and trace amounts evident at 180 min. The reason for the delayed onset of SB in the immature rat is unclear. The longer half-life is presumably related to incomplete development of enzyme systems metabolizing apomorphine in the immature animal.