Rat lateral and ventral prostate tissue was studied using ultrastructural and analytical techniques in adult animals castrated for periods of 3 to 20 days. As in previous reports involution of the prostatic epithelium following testosterone deprivation resulted in alterations of the amount, distribution, and conformation of the endoplasmic reticulum, Golgi, and lysosomes in addition to some nuclear changes. Although the fundamental changes were similar in both lobes there were specific effects on the individual lobes. Reductions in the subcellular levels of zinc were more pronounced in the lateral prostate, particularly with respect to the secretory and stromal concentrations of the metal. Despite low concentrations of circulating testosterone, secretory activity was still evident after a 20-day castration period and would appear to reflect a different sensitivity in the lateral lobe to the lowered testosterone level or stimulation by other hormones when compared to the ventral lobe. The significance of the subcellular distribution pattern of zinc during the experimental periods is discussed in relation to the possible functional significance of the metal in prostatic tissue.