Alfentanil 35 micrograms kg-1, was used successfully in a patient with severe aortic stenosis, in order to minimize the haemodynamic responses to intubation and surgery during Caesarean section. The baby was delivered apnoeic, unresponsive and with poor muscle tone, but responded rapidly to naloxone. Plasma alfentanil concentrations and percentage binding to plasma proteins were measured in both maternal and neonatal blood. Free drug concentrations were similar in both mother and neonate, but maternal plasma proteins had a higher affinity for alfentanil. Only 67.26% of neonatal plasma alfentanil was bound to plasma protein. This value did not differ significantly from those estimated from the blood of a further 12 healthy neonates.