delta 9-THC, sodium phenobarbital, or a combination of these drugs, were administered to pregnant mice on gestation days 6 to parturition. Levels of delta 9-THC in blood approximated levels attained in humans after smoking one to two marijuana cigarettes. Blood cannabinoid levels in mice were not significantly elevated by concurrent drug administration. Both drugs significantly reduced litter size and weight per pup at birth, but the combination of these drugs did not affect these outcomes to a significantly greater degree than either drug by itself. Phenobarbital but not delta 9-THC increased resorption rate.