Learned food aversions have been implicated in the anorexia which develops in rats with transplantable PW-739 tumors. There appear to be striking differences in growth characteristics and physiological effects of different experimental tumors. The present studies examined the issue of heterogeneity of tumor models, while assessing the generality of the finding that learned food aversions arise in anorexic, tumor-bearing animals. This was done by comparing effects on food intake and diet preferences of two transplantable tumors, the Leydig cell tumor, LTW(m), and the Walker-256 carcinosarcoma. We found that animals with Leydig tumors, like those with PW-739 tumors, developed strong aversions to the specific diet they had eaten after tumor implant. In contrast, animals with Walker-256 tumors did not develop diet aversions. These results support the idea that learned food aversions contribute to anorexia in animals with Leydig but not Walker tumors. They further suggest that learned food aversions in tumor-bearing animals are not a response to illness, in general, but rather that the unconditioned stimulus responsible for these aversions is quite specific, and may ultimately prove identifiable.