Ultrastructural morphometric studies were employed to compare the morphology of Leydig cells found in the testes of rats fed ethanol, accounting for 36% of total calories for 6 weeks, and those found in the testes of rats isocalorically fed a diet in which ethanol was isocalorically replaced with dextrimaltose. The testes of alcohol-fed rats weighed significantly less (p less than 0.01) than those of the isocaloric controls. Moreover, plasma testosterone in the alcohol-fed animals was reduced (p less than 0.01), as compared to the controls. Morphologically, two main differences were encountered between the Leydig cells of alcohol-fed animals and those of the isocalorically-fed controls. These were: (1) an increase in the number of elongated and cup-shaped mitochondria; and (2) an increase in cytoplasmic protrusions in the form of pseudopods. Using morphometric techniques, the Leydig cells of alcohol-fed animals were smaller (p less than 0.05), had less cytoplasm (p less than 0.05), larger mitochondria (p less than 0.01), and less smooth endoplasmic reticulum (p less than 0.05) than did those of the isocaloric controls. These morphologic characteristics of Leydig cells of alcohol-fed animals are similar to those reported to occur in the liver and suggest that the biochemical mechanisms responsible for alcohol-induced cellular injury are similar in the testes and the liver.