Pregnant mice were exposed to phenobarbital (PhB) on gestation days 9 to 18 (3 gm/kg milled food). Their offspring, who were exposed to the drug transplacentally (B offspring), were tested at an age of 50 days for apomorphine- (0.5, 1.0 and 2.0 mg/kg) induced hypothermia. At doses of 1.0 and 2.0 mg/kg, B offspring had less hypothermic response to apomorphine than controls (p less than 0.01); the effect was similar in both sexes. In order to acquire further understanding of the alterations in apomorphine hypothermia, mainly in relation to dopamine (DA) receptors, adult intact mice were exposed to haloperidol for 4 weeks (25 mg/kg milled food) in order to increase their DA receptor number, and their hypothermic response to apomorphine was tested 4 days post withdrawal. The treated animals had an increased DA receptor number, as was attested by a 23% increase in 3H-spiroperidol binding (P less than 0.01) and a 77% increase in apomorphine-induced climbing. However, their apomorphine-induced hypothermia did not differ from control. Therefore, there is no evidence as yet that alterations in apomorphine-induced hypothermia after prenatal exposure to PhB indicates changes in DA receptors, and the implications of this phenomenon still remain an open question.