Butorphanol kinetics were studied in 8 young (23-34 years of age) and 9 elderly (65-79 years of age) healthy male subjects following a single, intravenous 2-mg dose of butorphanol tartrate. Plasma and blood concentrations collected over 24 h were analyzed by a second antibody radioimmunoassay for unchanged drug. Systemic blood clearance (21.29 +/- 7.18 ml/min/kg), apparent steady-state volume of distribution (9.02 +/- 2.40 liters/kg), distribution half-life (7.67 +/- 4.66 min), and the blood/plasma concentration ratio (1.21 +/- 0.36) in the elderly did not differ from those in the younger subjects. However, the terminal elimination half-life was significantly prolonged in the elderly (5.51 +/- 2.49 versus 3.67 +/- 0.52 h, p less than 0.05). Positive correlations between age and terminal elimination half-life and volume of distribution were noted. Our data suggest that young and old subjects attain similar plasma concentrations after single doses; however, longer dosing intervals may be necessary for multiple dosing in the elderly. The appropriate dosing interval for butorphanol needs to be assessed in elderly patients.