To determine the effect of aging on the pharmacokinetics and pharmacodynamics of etomidate, we administered etomidate (5 to 10 mg/min) by intravenous infusion to 21 healthy surgical patients, age 22 to 82 yr. Etomidate produced progressive slowing of the EEG to an easily recognized pattern (stage 3) that determined the dosage endpoint. Subsequent power-spectrum analysis of the EEG gave the median frequency. Median frequency values and simultaneous measurements of blood etomidate concentration were incorporated into a sigmoid Emaxpharmacodynamic model that permitted an estimate of IC50, the blood etomidate concentration which produced a 50% reduction in the median frequency. The dose of etomidate required to reach the uniform EEG endpoint decreased significantly with increasing age (r2= .68) as did the dose needed to produce maximal median frequency depression (r2= .69). None of the parameters of the pharmacodynamic effect model, including IC50, correlated with age, suggesting that increased brain sensitivity in the elderly does not cause the age-related change in dose requirement. The initial distribution volume for etomidate decreased significantly with increasing age (r = .56), implying that a higher initial blood concentration in the elderly following any given dose of etomidate is part of the cause of the lower dose requirement in the elderly patient. A contracted initial distribution volume in the elderly may result from well described physiologic changes of age. Etomidate clearance also decreased with age. Age-dependent changes in etomidate pharmacokinetics rather than altered brain responsiveness may be the basis for the decreased etomidate dose requirement in the elderly.