HS/Ibg (heterogeneous stock) mice dams were fed milled mouse food containing 3 g/kg phenobarbital (PhB) in acid form and water as their only nutritional source from gestation days 9--19. Control females received milled food and water. Blood PhB levels of treated females and fetuses were 40--200 micrograms/ml blood. At the age of 50 days, male offspring were injected with C14 -sodium pentobarbital (PenB) (50 mg/kg). Sleep time and temperature loss were monitored and, in randomly selected individuals, brain PenB levels were determined upon awakening. The experiment was repeated on the same animals for 3 consecutive days. All offspring developed functional (central nervous system) tolerance during the 3 testing days as evidenced by the daily decrease in sleep time while brain levels of PenB upon awakening increased (P less than 0.001). Offspring who received PhB prenatally had generally shorter sleep times, less temperature loss, and higher brain PenB levels upon awakening than controls. The differences were most pronounced on the second day (sleep time reduced 27%, P less than 0.001; temperature loss 47%, P less than 0.001; brain PhB levels increased 23%, P less than 0.01).