CCAAT/enhancer binding proteins alpha and beta (CEBPA/ CEBPB) were evaluated in the porcine ovary during the estrous cycle. CEBPB mRNA was present in antral follicles and was significantly increased in healthy corpora lutea (CL), whereas CEBPA mRNA was constitutively expressed in these structures. Both isoforms of CEBPA (42 and 30 kDa) exhibited greater expression in preovulatory follicles, and the 42-kDa isoform increased in CL, whereas the 30-kDa isoform decreased. All major isoforms of CEBPB (38, 34, and 20 kDa) were expressed, with the 34- and 20-kDa isoforms being more abundant in preovulatory follicles and further increased in CL. The effects of FSH and cAMP analogue on the distribution of CEBP isoforms were evaluated in primary cultures of porcine granulosa cells. FSH and 8-Br-cAMP had little stimulatory effect on isoform distribution, but cAMP treatment for 24 h tended to decrease the 30-kDa form of CEBPA and the 34-kDa form of CEBPB. The 34-kDa form of CEBPB was decreased by the protein kinase A inhibitor H89 at 4 h (with FSH treatment), and by both protein kinase A and phosphatidylinositol 3-kinase inhibitors at 24 h of treatment. In transfected granulosa cells, FSH and cAMP analogue stimulated a CEBP consensus sequence-reporter construct that was blocked by H89. These data implicate protein kinase A as the major regulator of CEBPB isoform distribution and CEBP-mediated transactivation in granulosa cells. The differential expression of specific CEBPA/B isoforms observed in maturing follicles and CL may contribute to changes in follicular cell differentiation and increasing steroidogenic capacity.