Adenosine 3',5'-monophosphate (cyclic AMP) receptor protein of 56,000 daltons increases markedly in mammary tumors induced by 7,12-dimethylbenz[a]anthracene (DMBA) after incubation of tumor slices with cyclic AMP, benzamide, and arginine. Incubation of cytosol from these tumor slices with nuclei from unincubated tumors results in nuclear uptake of the 56,000-dalton cyclic AMP receptor and in phosphorylation of the 76,000-dalton nuclear protein. Binding of the 56,000-dalton receptor and phosphorylation of the 76,000-dalton protein also occur in DMBA tumor nuclei when protein kinase type II of bovine heart is used. The results suggest that cyclic AMP receptor is involved in the nuclear events of a hormone-dependent mammary tumor.