Recent work of Green et al. (Gr75) has drawn attention to the potential risk for genetic damage to gonadal tissue due to the nonhomogenous distribution of internal emitters like plutonium. We have recently undertaken a quantitative autoradiographic study in mouse testes of the microdistribution of 239Pu, the kinetics of its translocation, and the consequences of nonuniform gonadal irradiation. Twenty-six B6CF1 male mice were injected intravenously with 10 micro 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 approx. 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%), we calculate that at least half the alpha energy from the interstitial tissue Pu must be absorbed by spermatogonial stem cells. Accordingly, our data indicate that 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. We also suggest the possibility of impaired androgen production due to the relatively high concentration of Pu long retained in the interstitial tissue.