In two separate experiments (C3H x 101)F1 female mice were injected intravenously with 239Pu in trisodium citrate, then mated in pairs to strain CBA males, to test for dominant lethality. In the first experiment 10 muCi kg-1 and in the second 20 muCi kg-1 body mass was injected. Matings were after 6 days in the first experiment (estimated ovarian absorbed dose of 0.1 Gy) and after 3, 6 or 12 weeks in the second (estimated ovarian doses of 1.11, 2.45 and 5.91 Gy respectively). No evidence of dominant lethal induction was found in the first experiment, but in the second there was a significant increase over controls in pre-implantation loss in all three series. Post-implantation lethality increased significantly (by 12 per cent) only after 12 weeks' exposure. With the 6- and 12-week exposures (especially the latter) luteal counts fell and fewer females became pregnant than in controls. This is attributed to oocyte killing by the alpha-particles. Histological and autoradiographic investigations showed a marked reduction in ovarian size and follicular numbers with fission-tracks clustered mainly over the medullary stroma. The pre-implantation loss may stem from lowered fertilization of oocytes because of their damage, so that the best measure of dominant lethality is that based on post-implantation death. Thus there is only slight evidence for the induction of genetic damage, which is in line with previous findings after chronic exposures of female mice.