The morphogenesis of the Leydig cells and germ cells of the pig during the first 130 days after birth is described in relation to estimates of cellular radiosensitivity. Boars aged 1 to 30 days were exposed totally to 200 r of gamma rays and the effects on the Leydig cells and germ cells were determined before and after puberty. The number of gonocytes (primitive germ cells) declined to 10% of the control values by Day 30 after irradiation, but at Day 60 some recovery was evident. Postpubertally, an average of 23% of the seminiferous tubules seen in transverse section in the irradiated testes exhibited radiation damage which was presumably permanent. Average sperm production was 47% of the control value. Increased numbers of crenated nuclei and a delay in morphogenesis were noted among the Leydig cells before puberty. Urinary steroid output of the irradiated boars equalled or exceeded that of the non-irradiated boars and it is assumed that 200 r of gamma rays effected no permanent damage among the Leydig cells.