Twenty-one women about to undergo elective Caesarean section were given intravenous alfentanil 10 micrograms kg-1 1 min prior to induction of anaesthesia in order to obtund the pressor response to laryngoscopy and endotracheal intubation. Compared with a control group of 16 patients, alfentanil significantly reduced the pressor response to endotracheal intubation (P less than 0.01), without any detectable adverse effect upon the neonate. At delivery, mean maternal alfentanil plasma concentration was 23.5 ng ml-1 (SD 7.5, range 9.8-41.2 ng ml-1) and the mean maternal venous plasma alpha 1-acid glycoprotein (alpha 1-AGP) was 576 mg l-1 (SD 208, range 230-1200 mg l-1). Mean neonatal umbilical venous alfentanil plasma concentration was 7.5 ng ml-1 (SD 1.8, range 3.6-10.6 ng ml-1), while the mean umbilical venous alpha 1-AGP concentration was 189 mg l-1 (SD 76, range 80-410 mg l-1). At delivery, the calculated unbound maternal and foetal plasma alfentanil concentrations were similar.