The CCAAT/enhancer binding proteins (C/EBPs) are a highly conserved family of DNA binding proteins implicated in the transcriptional control of genes involved in cell growth and differentiation in a variety of tissues. The expression of C/EBP-alpha, beta, and delta mRNA in the normal mouse mammary gland was investigated during pregnancy, lactation, and involution via Northern blotting and in situ hybridization. Mammary gland C/EBP-alpha mRNA is detectable at low levels during pregnancy and postlactational involution. C/EBP-beta mRNA levels are elevated during pregnancy, decline slightly in midlactation, and are induced within 48 hours of the onset of involution. C/EBP-delta mRNA content is low throughout pregnancy and lactation, but increases dramatically (>100-fold) within 12 hours after the onset of postlactational involution. In situ hybridization demonstrates that mammary epithelial cells are responsible for the expression of C/EBP-delta mRNA during involution. In contrast to mammary gland, C/EBP-alpha is the predominate isoform expressed in liver with relatively low expression of C/EBP-beta and C/EBP-delta mRNA. Liver C/EBP isoform mRNA levels are unaffected by lactation status. These results demonstrate the tissue-specific regulation of C/EBPs. The pronounced sequential induction of C/EBP-delta and C/EBP-beta during postlactational involution is consistent with a role for C/EBPs in the regulation of mammary epithelial cell apoptosis.