Rats were given ⁶⁵ZnCl₂ and their internal organs were examined by autoradiography. It was thereby found that ⁶⁵Zn is deposited in high concentration in the prostate gland. The prostate was taken out, homogenized, and the homogenate was centrifuged by which majority of the radioactive zinc (70%) transited to the water-soluble portion. The water-soluble peptide was submitted to paper chromatography, using butanol-acetic acid-water (4:1:3) as the solvent and a spot appeared at around Rf 0.25 which colored bright red to ninhydrin and pinkish violet to dithizon. By scanning for the radioactive zinc on this chromatogram, the presence of radioactive zinc was detected at Rf 0.25, and the ultraviolet spectrum of this portion indicated absorption maximum at 262mμ. The same result was obtained on administration of ⁶⁵Zn-histidine to the rat. Paper chromatography of the homogenate of the prostate from ordinary rat hardly gives any spot at around Rf 0.25 which colors red to ninhydrin but the spot colors slightly with dithizon. Ultraviolet spectrum of this portion shows a slight absorption at 260-262mμ. The hydrolyzate of the water-soluble peptide with hydrochloric acid gave one spot on paper chromatogram at Rf 0.27 for zinc-bound amino acid and this spot is that of Zn-histidine with absorption maximum at 262mμ.