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Doxacurium (30 ig/kg) pharmacokinetics and pharmacodynamics were evaluated in nine elderly (age range, 70 to 83 years) and nine young (age range, 19 to 39 years) patients under nitrous oxideisoflurane anesthesia. The force of contraction of the adductor pollicis was monitored and plasma samples were collected for an 8-hour period. In the elderly group, doxacurium elimination half-life was prolonged (119.7 versus 75.9 minutes) and plasma clearance was significantly reduced (1.75 versus 2.54 ml/min/kg) without any change in volume of distribution. Onset (12.9 versus 8.9 minutes) and recovery times (113.4 versus 48.1 minutes) were longer in the elderly group. The equilibrium rate constant to the effect compartment (lc.) was decreased in the elderly (0.039 versus 0.051 min -1), whereas the effect compartment concentration at 50% block was similar in both groups (44.7 versus 54.1 ng/ml). An agerelated reduction in muscle blood flow may be responsible for the decrease in keo. The pharmacokinetic changes observed in the elderly are consistent with a decreased function in the organs of elimination.Differences in response to drugs between young and elderly patients have been frequently documented. Decreased cardiac output, renal and hepatic blood flow, glomerular filtration rate, and hepatic function attributable to aging may modify the pharmacokinetics of drugs. [1][2][3][4][5] For neuromuscular-blocking agents, the most common features are an increased onset time, a prolonged effect, and a decreased elimination of the drug in the elderly. Such changes have been observed for pancuronium,6'7 atracurium,8 vecuronium," alcuronium,11 tubocurarine, and metocurine.12 Recently the pharmacokinetics and pharmacodynamics of doxacurium, a new long acting nondepolarizing blocker, were shown to be unchanged in elderly patients.13'14 Though the disposition of doxacurium in humans has not been fully characterized, preliminary studies indicate that it would be excreted by way of both urinary and biliary pathways .13'15-16 Therefore a decreased