Metallothionein in the rat prostate was demonstrated by using an indirect immunoperoxidase technique. Strong staining was found mainly in the supranuclear area of the cytoplasm of the glandular epithelium in the dorsal and the lateral glands. Diffuse cytoplasmic staining and nuclear staining were found with less frequency. Secretory granules were found in the lumen with various intensities of staining. Most of the epithelial cells in the ventral lobes and the coagulating glands were negatively stained. Immunoreactive cells were found sporadically. Two types of immunoreactive cells, round cells and spindle-shaped cells, were scattered in the stroma. Vascular endothelial cells, red blood cells, and smooth muscle cells were negatively stained. These results suggest the synthesis and secretion of metallothionein in the rat prostate.