Morphine-3-glucuronide (M3G) is the major metabolite of morphine and is present in the circulation of persons treated with morphine or abusing heroin. This project was designed to study the kinetics of M3G in the foeto-maternal compartment, since this metabolite may be of relevance for the abstinence syndrome observed in neonates of pregnant abusers. The kinetics of M3G were studied in two non-pregnant and four pregnant Rhesus monkeys. M3G was given as a bolus injection in four of the animals and as a long-term infusion for 12 hr in two animals. M3G passed slowly across the placenta to the foetus and amniotic fluid. After 10 hr of M3G infusion, the foetal plasma M3G concentration was measured in two cases and found to be 37% and 72%, respectively, of the maternal concentration.