SUMMARY 1. Autografts and intra- and inter-strain homografts of ovarian tissue were made among rats of the two strains, albino and hooded, maintained at the National Institute for Medical Research. 2. Young adult females were ovariectomized and ovarian tissue transplanted subcutaneously. Surviving animals were killed 12–13 months after operation and the grafts, where present, removed for histological examination. 3. The occurrence of vaginal cornification was taken to indicate functional activity on the part of the graft. The effectiveness of the type of graft was assessed by the proportion of 'takes' within a group, by the average latent interval before the appearance of vaginal cornification and by the functional survival time of the graft. 4. The proportion of 'takes' was maximal in the autografts and intra-strain homografts, and only slightly less (16/20) in the inter-strain homografts. The length of the latent interval was not significantly different with the six types of graft. After 12–13 months, when the animals were killed, all the autografts, 11/14 of the intra-strain homografts, and 7/18 of the inter-strain homografts were still functioning. Of the thirty-six homografts which became established sufficiently to cause vaginal cornification, nine failed during the 1st month and three only during the 2nd and 3rd months. There were no later failures. 5. Histologically, most of the grafts were characterized by large follicular or lutein cysts, but normal eggs, follicles and corpora lutea were found in grafts of each type. There was a close connexion between the occurrence of follicular cysts and of persistent vaginal cornification. 6. The comparative importance of immunological and endocrinological factors in the evolution of the ovarian homograft is discussed.