Alteration of growth of dimethylbenz[a]anthracene-induced mammary tumors was caused by removal of estrogen (ovariectomy), or insulin (diabetes), or by inhibition of prolactin secretin (treatment with an ergoline derivative). The levels of cyclic AMP (cAMP) and cGMP were measured in carcinomas classified as growing, static, and regressing. The amount of cAMP, expressed as pmoles/mg tumor weight or pmoles/mg protein, was lowest in growing tumors, intermediate in static tumors, and highest in those regressing. No correlation was seen between tumor growth and cGMP levels. Cyclophosphamide-induced tumor stasis did not elevate cAMP levels. The data suggest a role of cAMP in arrest of hormone-induced tumor growth.