Pregnant dogs were administered 0.5 mg/kg of 3H-.DELTA.9-tetrahydrocannabinol i.v. and sacrificed at 30 min, the time of peak behavioral effects. The distribution of radioactivity in mothers and fetuses was quantified in many peripheral tissues and in major brain areas by sample combustion and liquid scintillation spectrometry. Brains were subfractionated by differential centrifugation to compare the subcellular localization of radioactivity in maternal and fetal brains. Although the level of radioactivity in maternal brain was 3 times higher than that in fetal brain, the distribution of radioactivity in the major brain areas was similar in mothers and fetuses. The subcellular distribution of radioactivity in whole brain homogenate of fetal dogs was markedly different from that in whole brain homogenates from mothers and appeared to be related directly to differing phospholipid content.