Recent work of Green et al has drawn attention to the potential risk for genetic damage to gonadal tissue due to the nonhomogenous distribution of internal emitters like Pu. A quantitative autoradiographic study in mouse testes of the microdistribution of 239Pu, the kinetics of its translocation, and the consequences of nonuniform gonadal irradiation was undertaken. B6CF1 male mice (26) were injected i.v. with 10 .mu.Ci/kg of monomeric 239Pu-citrate. Groups of mice were sequentially sacrificed at 6, 30, 75, 222 and 348 days following Pu injection, and testes were weighed and assayed for Pu. At each time point, testes were processed and examined by standard histological and quantitative autoradiographic techniques. Between 6 and 348 days after injection, the overall Pu concentration in the testes, as determined radiochemically, increased by approximately 40%, largely due to a reduction of 30% in the average testis weight. Autoradiographic alpha track counts made over this period showed plutonium deposition mainly in and adjacent to the basement membranes of the seminiferous tubules (52% of the tracks), and in the interstitial tissue (41%). Because of the small fraction of the testis volume occupied by interstitial tissue (about 5%), at least half the alpha energy from the interstitial tissue Pu must be absorbed by spermatogonial stem cells. The factor of 2.5, proposed for Pu by Green et al to convert mean testis radiation dose to the dose received by the stem cells, should be increased to at least 4. The possibility of impaired androgen production due to the relatively high concentration of Pu long retained in the interstitial tissue was suggested.