Our prior studies on liver growth during late gestation in the rat have shown that hepatocyte proliferation is developmentally modulated; it declines as term approaches, resumes in the immediate postnatal period, and declines during the latter part of the first postnatal week as hepatocytes attain an adult phenotype. The present studies were designed to elucidate mechanisms accounting for both temporary perinatal hepatocyte growth arrest and subsequent progression to the low level of cell growth seen in adult liver. Our studies focused on the family of MAP kinase signaling pathways as well as downstream cell cycle regulation. Key signaling kinases were examined for the ontogeny of their activity during late gestation and early postnatal liver development in the rat. Prior studies had shown that the prototypical MAP kinase pathway, which includes the 42 and 44 kDa MAP kinases (Erk1 and Erk2), is inactive in developing hepatocytes in vivo. We now report that the second MAP kinase pathway, involving the family of Jun N-terminal kinases(JNKs), is constitutively active in fetal liver and inactive in adult liver. However, JNK activity did not correlate with cell growth; activities in preterm and term liver were similar. The profile of JNK activity was recapitulated in gel shift assays for DNA binding activity to an AP-1 site in the liver collagenase gene promoter; nuclear extract AP-1 binding activity was high in both preterm and term fetuses, and declined after the first postnatal week. The third MAP kinase pathway, for which p38/HOG1 is the terminal kinase, was found to be inactive in livers obtained on days 17 and 19 of gestation. However, p38 was active in term (21 day) liver. It was again inactive in the immediate postnatal period, but activity increased during the first two postnatal weeks to the high level seen in adult rat hepatocytes. This activity pattern was inversely related to liver nuclear extract content of cyclin D1(determined by Western immunoblotting). Of note, p38 has been shown to regulate cyclin D1 expression in other systems. In addition, p38 is known to be regulated by cellular stresses such as change in redox state, osmolar stress and heat shock. We conclude from these studies that perinatal“stress” may result in hepatocyte growth arrest, and that this temporary period of quiescence is mediated by stimulation of p38 (but not JNK) activity which, in turn, suppresses expression of a key G1-phase cyclin.