The effects of castration and fasting upon the alcohol elimination rate, liver alcohol dehydrogenase (LADH) maximum activity (Vmax), and hepatic concentrations of ethanol, acetaldehyde, and free NADH during ethanol oxidation were examined in male Wistar rats. Castration increased the Vmax of LADH and, to a lesser extent, the alcohol elimination rate in vivo. On the other hand, fasting reduced the Vmax of LADH and the alcohol elimination rate in sham-operated and castrated rats but it did not nullify the effect of castration. Castration produced small but significant changes in the hepatic concentrations of ethanol, acetaldehyde and free NADH in fed rats during ethanol oxidation. Fasting also caused significant increases in the concentration of free NADH during alcohol oxidation in both the sham-operated and castrated groups. The ratio of the steady-state velocities of LADH in situ to the maximum velocities of LADH (v/Vmax) under the different experimental conditions was calculated by using the steady-state rate equation for the enzyme mechanism of rat LADH and its kinetic constants. The calculated v/Vmax ratios were 50-62%, indicating that LADH activity was limited to about the same extent by its substrates and products under these conditions and that the changes in alcohol elimination rates produced by fasting and castration mainly reflected changes in the Vmax of LADH. The calculated steady-state velocities in situ (v) were 14-28% lower than the measured rates of alcohol elimination in vivo. The extent of agreement is probably acceptable in view of the assumptions needed to determine the free NADH concentration in liver and the existence of non-LADH-related processes for alcohol elimination in vivo.