The DNA excision repair ability of UV-irradiated male germ cells of dog and rat was determined by silver grain counting after ,3,H-TdR incorporation and autoradiography. The relative content of alkaline-toluidine blue O-stained non-histone nuclear proteins (NHP) ,in situ, of the normal germ cells was measured by microspectrophotometry. Among the germ cells the primary spermatocytes contain the most NHP, they have revealed the highest repair ability; the spermatogonia contain less NHP, a lower repair ability (but that of dog spermatogonia was only a slightly lower average value); the round spermatids contain even less NHP, they, as well as the spermatozoa, no repair ability. A significant statistical correlation coefficient between the repair ability and the nuclear NHP content had been obtained in dog germ cells; while that in the rat germ cells has not yet been obtained, merely owing to the technical difficulty in measuring the NHP content in the irregular-shaped spermatozoan nuclei. By the microscopical observation or together with the microspectrophotometry, two groups of spermatozoa had been identified in dog and rat testes: the one's nuclei contain a little amount of NHP and the other's don't. No NHP was found in the nuclei of dog spermatozoa taken from ejaculated semen.