The transplacental transfer of morphine and morphine-3-glucuronide (M3G) was studied in five cases of suspected Rh-isoimmunization. Ultrasound-guided fetal blood sampling from the umbilical vein was carried out as a diagnostic procedure before intrauterine blood transfusion. Morphine was given as a parenteral premedication to the mother at a dose of 0.13-0.20 mg/kg bw. Fetal blood was sampled 5-74 minutes after the morphine administration. These five women were investigated on 14 different occasions. The plasma concentrations of morphine and M3G were measured in blood samples collected simultaneously from mother and fetus. The feto-maternal ratio of morphine was 0.96 at five minutes and remained close to 1.0 in most samples taken later. At 12 minutes the ratio of M3G was less than 0.002 and between 0.2 and 0.6 in the later samples. The feto-maternal plasma ratios of morphine and M3G did not change over the studied period in one woman investigated five times between gestational weeks 26 and 32. This is the first time transplacental transfer of morphine has been quantified in man. Our results demonstrate a rapid transplacental passage and equilibration of morphine between mother and fetus.