The effects of age on the pharmacokinetic and pharmacodynamic responses to rocuronium (Org 9426) were studied in 20 elderly (>70 yr) and 20 younger control patients (<60 yr) during N2O/O2, fentanyl anesthesia. The onset times were the same for both the elderly and younger control group, but the duration of action of rocuronium was significantly prolonged in the elderly patients. Elderly patients, when compared with the younger, also exhibited a significant decrease in plasma clearance (3.67 ± 1.0 vs 5.03 ± 1.5 mL·kg−1·min−1, mean ± SD) and volume of distribution (399 ± 122 vs 553 ± 279 mL/kg, mean ± SD). During the recovery phase of paralysis, no significant difference was seen in the log plasma concentration versus twitch tension response relationship between 20% and 80% paralysis in young and elderly patients receiving rocuronium. The differences in action of rocuronium between the elderly and younger groups can be fully explained by the observed differences in the distribution and elimination of rocuronium between the two groups. The decreased total body water and decreased liver mass which normally accompany aging are likely explanations for the pharmacokinetic changes found in the elderly in this study. We conclude that the action of rocuronium is prolonged in patients aged more than 70 yr because of decreased elimination of the drug.