The role of the endoplasmic reticulum stress-regulated kinase, PERK, in mammary gland function was assessed through generation of a targeted deletion in mammary epithelium. Characterization revealed that PERK is required for functional maturation of milk-secreting mammary epithelial cells. PERK-dependent signaling contributes to lipogenic differentiation in mammary epithelium, and perk deletion inhibits the sustained expression of lipogenic enzymes FAS, ACL, and SCD1. As a result, mammary tissue has reduced lipid content and the milk produced has altered lipid composition, resulting in attenuated pup growth. Consistent with PERK-dependent regulation of the lipogenic pathway, loss of PERK inhibits expression of FAS, ACL, and SCD1 in immortalized murine embryonic fibroblasts when cultured under conditions favoring adipocyte differentiation. These findings implicate PERK as a physiologically relevant regulator of the lipogenic pathway.