The perfect analgesic regimen is constantly sought, no matter how labor is conducted. The quest for an effective drug that will afford maximum relaxation and pain relief with minimum interruption of any natural homeostatic mechanism is a foremost subject in present obstetric analgesics research. Synthetic alternatives are being offered, promising perfect compatibility with the clinician's demands. Nalbuphine, a semisynthetic narcotic agonist-antagonist analgesic of the penanthren series, is supposed not to be liable to cause respiratory depression and is expected to have fewer side effects. A double-blind, randomised prospective study of 137 patients who received 10 mg nalbuphine or 50 mg pethidine i.v. during the active phase of labor in term was carried out. Maternal cardiovascular variables, pain intensity, progress of labor and fetal heart rate during labor were related to side effect and neonatal outcome (1- and 5-min Apgar scores and umbilical venous pH). Neither regimen showed an advantage over the other. Data analysis points to a possible transient depressive effect induced by nalbuphine on the fetal or neonatal central nervous system.