The distribution of halothane-82Br in maternal and foetal tissues was studied in near-term pregnant guineapigs exposed to 0.5% concentration of the vapour for periods varying from 5 to 45 min. At all times observed, the specific activity of the 82Br label was higher in maternal brain, lung and kidney than in the corresponding foetal tissues. In contrast, specific activity was slightly higher in foetal than in maternal liver up to 20 min and more than three times greater at 30 and at 45 min. The uptake of halothane by foetal liver in vitro was more than double that of maternal liver; in maternal and foetal brain the uptake was identical. The foetal liver plays a significant role in decreasing the amount of halothane reaching the foetal brain from the maternal circulation.