Xenografting of ovarian tissue into immuno-deficient mice is useful in studying the dynamics of follicular development. We have demonstrated that xenografted bovine secondary follicles develop to the antral stage in female severe combined immunodeficient (SCID) mice. We did this by examining the development of bovine secondary follicles (140-190 μm in diameter) that had been grafted into male and female SCID mice for 4 and 6 weeks. We then compared the results for the two groups. The rate of surviving follicles in the grafts was similar in male and female mice, but the survival rate of oocytes was lower in male mice in these follicles, especially the antral follicles. In addition, the basement membranes of relatively large follicles were thinner and torn in the male mice, and erythrocytes had invaded the follicular cavity. The mean diameters of surviving follicles and oocytes were significantly larger in both male and female mice than before grafting. In female mice, the diameter of antral follicles increased gradually as the grafting was prolonged, although the difference was not in significant. Surviving oocytes in the follicles increased in diameter. In contrast, development of antral follicles in male mice seemed to be accelerated, but, in contrast to female mice, the mean diameters of antral follicles and surviving oocytes showed no further increase after 4 weeks of grafting. These results suggest that bovine follicles can develop in male SCID mice, but oocyte degeneration together with the follicular degeneration occurs in large antral follicles at a higher rate in males than in the females.