The pharmacodynamics of alfentanil given as a supplement to nitrous oxide anaesthesia were studied in 10 elderly patients undergoing lower abdominal surgery. Plasma alfentanil concentration-effect curves for perioperative stimuli and for the probability of needing naloxone at the end of surgery were fitted by logistic regression. The concentration at which the probability of no response was 0.5 (Cp50) for laryngoscopy was 356 ng ml-1, for opening the peritoneum 295 ng ml-1 and for closing the peritoneum 254 ng ml-1. The average Cp50 for the intra-abdominal part of the operation was 268 +/- 97 ng ml-1. The Cp50 for needing naloxone was 174 ng ml-1. These findings are similar to published values for young healthy females undergoing lower abdominal surgery. Our findings suggest that the pharmacodynamics of alfentanil in the elderly are similar to those in younger adults.