The kinetic disposition of morphine was compared in 13 young and 7 older healthy subjects after a single intravenous dose of 10 mg/70 kg morphine. The apparent volume of distribution at steady state in elderly subjects was only half of that in young subjects. This difference was derived from reductions in both central and peripheral kinetic compartment volumes in the older subjects. The beta elimination phase for morphine was more rapid, but its plasma clearance was reduced in older subjects. Calculation of the peripheral compartment morphine concentration for these subjects indicated that drug concentration in this kinetic space was higher in older subjects for 1.5 hr after dosing.