Maternal and umbilical venous serum naloxone concentrations were measured after the i.v. (30 mothers) or i.m. (seven mothers) injection of naloxone 400 micrograms. Following i.v. naloxone there was rapid transfer to the fetus and therapeutic plasma concentrations could be expected in 1 or 2 min. However, following i.m. administration, neonatal concentrations were variable. In five non-pregnant volunteers naloxone 400 micrograms i.v. resulted in concentrations greater than 2.5 micrograms ml-1 for up to 30 min. After naloxone 2 mg i.m., a peak was reached at 10 min, followed by slow clearance over several hours.