The F, generation hybrids of carp and goldfish exhibited a marked hyperplasia of pars distalis basophil (gonadotrophic) cells, which first became evident in 3-to 4-year-old fish, the age at which carp and goldfish normally become sexually mature. Many of these hybrids also developed gonadal tumors, the genesis of which also coincided with the onset of gonadotroph proliferation. In these fish the baso phil hyperplasia was more marked than in nontumorous hy brids. Tumor-bearing carp also

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... gonadotroph hy perplasia similar to that in tumorous hybrids. Both tumorbearing and nontumorous hybrids showed a marked reduc tion in gonadotroph cell number when fed a methallibure diet, although there was no apparent change in either the gross pathology or the histopathology of the tumors. These findings suggest that the hyperplasia of the gonadotrophs in tumorous fish is not, ;'pso facto, responsible for the maintenance of the tumors. On the contrary, since gonado troph hyperplasia was evident in both tumorous and nontu morous hybrids (all of which were sterile), it may be sympto matic of the sterile condition of the fish rather than of the tumor per se. Evidence for the presence of a second gonadotrophic cell type in cyprinids is presented and discussed. Other pituitary cell types were similar in structure and ultrastructure in carp, goldfish, and hybrids and were unaffected by methallibure treatment.