The androgen-dependent NB rat prostate adenocarcinoma 2PR 129 and the autonomous tumor 102PR were transplanted into groups of both male and female nude mice, nude rats, and NB rats. Significant growth of the 2PR 129 tumor at the site of transplantation was noted in male but not female nude mice, nude rats or NB rats. Indeed, successful transplantation of 2PR 129 was not achieved in female nude mice even with usage of antilymphocyte sera. In contrast, the autonomous tumor 102PR grew at a significant rate at the local site of transplantation in mice and rats of both sexes. Although significant tumor masses were found at the injection site of male nude mice transplanted with 2PR 129 and in both male and female nude mice injected with 102PR, metastasis was not found. Further, antilymphocyte sera increased the rate of growth of both tumors in nude mice but did not change the apparent metastatic potential. Nonetheless, metastasis was readily observed in male nude rats transplanted with 2PR 129 and nude rats of both sexes transplanted with 102PR. Such metastatic lesions of nude rats were found in lung, liver and kidney with a similar frequency and histologic profile as NB rats. These results suggest that the nude rat, at least for this rat tumor system, is a more versatile and biologically relevant system than nude mice and further cautions against use of xenotransplants as models of tumor natural history. Moreover, it suggests the possible relationship between host recognition of species-determined genetic loci and metastatic potential.