Activation of the hypothalamo-pituitary-adrenal (HPA) axis during stress is associated with increased expression of genes that code for regulatory hormones such as corticotrophin-releasing factor (CRF) and ACTH. The identity of the transcription factors that mediate these changes in gene expression is not known. In the present study we have investigated the expression of the cAMP response-element binding protein (CREB) in mouse pituitary, and its regulation during a pharmacological paradigm that simulates activation of the CRF-ACTH axis. Using Western blots and DNA binding assays we have shown that both CREB protein (43 kDa) and CRE binding exhibit a readily-detectable basal level of activity in the pituitary. Following treatment with the 11 beta-hydroxylase inhibitor metyrapone, CRE binding activity was increased at 1 and 2 h but levels of CREB protein were not found to be consistently elevated. However, using a Ser133 phosphopeptide-specific antibody, that detects the functionally important phosphorylated form of CREB (P-CREB), we have shown that levels of pituitary P-CREB are markedly elevated following metyrapone. The same antibody was also used in DNA binding assays, and in the presence of this antiserum CRE binding activity in samples extracted from metyrapone-treated animals was reduced to levels similar to controls. Parallel experiments have confirmed previous studies showing increases in c-Fos expression and AP-1 DNA binding activity following metyrapone treatment but we have shown that c-Fos-associated binding activity does not appear to contribute to the increase in activity detected using the CRE binding probe. Our evidence of functionally relevant changes in pituitary CREB activity following glucocorticoid depletion must be viewed in the context of numerous other novel pituitary transcription factors that are implicated in HPA regulation, but our use of mice as an experimental model has facilitated the use of novel mouse mutants that can be used to dissect the role of individual factors.