Alfentanil [an analgesic] disposition after an i.v. bolus of 50 .mu.g/kg was followed in 15 elderly surgical patients and was compared to that in 9 young adults. A 2-compartment open model described alfentanil disappearance from plasma. Apparent volumes of distribution of the central compartment (201 and 211 ml/kg; .hivin.X), at steady-state (460 and 543 ml/kg), and of the AUC [area under the concentration-time curve] (746 and 722 ml/kg) in young adults and in elderly subjects did not differ. Plasma clearance was lower in elderly subjects (4.4 ml/min per kg) than in young adults (6.5 ml/min per kg), whereas terminal plasma t1/2 [half-life] was longer in the elderly patients (137 and 83 min). Alfentanil dosage should evidently be reduced in elderly patients when large single doses, multiple doses or long-term infusions are required.