The mammary gland provides an excellent system for investigating factors involved in the regulation of epithelial growth, differentiation, morphogenesis, and involution. We have recently demonstrated that clusterin gene expression is associated with epithelial differentiation and morphogenesis during murine embryogenesis. In the present study, we have analyzed expression of clusterin in the rat mammary gland with particular emphasis on the periods of pregnancy and lactation. By Northern blot analysis we show that clusterin mRNA is present in virgin rat mammary glands, that it is increased during pregnancy and strongly down-regulated during lactation, and that mRNA levels return progressively to those found in virgin rats during the course of involution. Total protein levels, as assessed by Western blot analysis, were also seen to decrease during lactation. In situ hybridization showed that strong expression of clusterin was localized to epithelial cells of developing lactiferous ducts and alveoli during pregnancy, and confirmed that epithelial clusterin expression is down-regulated during lactation. This down-regulation was reproducibly observed in mothers to which pups were returned after 2 days of weaning, suggesting that maternal mammary clusterin production is under hormonal control. These results show that clusterin gene expression is highest in alveolar and tubular epithelial cells during pregnancy and lowest during lactation, when the protein synthetic machinery of differentiated alveolar epithelial cells is devoted to milk production. This suggests that clusterin may be involved not only in mammary gland involution but also in the regulation of tubuloalveolar morphogenesis and alveolar epithelial cell differentiation in the adult rat mammary gland.