Protein synthesis by isolated polyribosomes from the dog prostate and the dorsolateral lobes of the rat prostate, in the presence of pH 5 fractions from the same, or different tissues, is stimulated by low concentrations (10-25 μM) of exogenous Zn2+. This effect of Zn2+, which can be duplicated partially by Cd2+, is unlikely to be due to a direct requirement for the cation at the polyribosomal level. Both the mature dog prostate and the dorsolateral lobes of the rat prostate contain a high concentration of Zn2+ (about 130 and 210 μg Zn2+/g wet wt tissue, respectively) approximately 50 % of which is associated with the cell sap fraction of the tissue. This soluble Zn2+ is not co-precipitated with protein on preparation of pH 5 fractions. When these Zn2+-rich homologous cell sap fractions are used as sources of activating enzymes, additional Zn2+ is inhibitory to protein synthesis by polyribosomes from both the dog prostate and the rat dorsolateral prostatic tissue. Since polyribosomes of these organs function in vivo in environments in which the concentration of Zn2+ (in various forms) is high, it is probable that with the isolated particles in vitro the natural conditions are stimulated more closely by the Zn2+-rich cell sap fractions, and that the same result is achieved by the addition of Zn2+ to systems that contain the Zn2+-deficient pH 5 enzyme fractions. © 1973.