Expression of annexin-I was investigated in the rat mammary gland during pregnancy, lactation, and involution. Both the mRNA and protein were very abundant in the mature virgin gland, but declined significantly by midpregnancy. In the lactating gland, little or no annexin-I was detected. After weaning, mRNA and protein levels increased dramatically, corresponding to glandular involution. We also show that premature removal of the suckling stimulus caused a rapid increase in mRNA expression, but that translation of message was delayed, possibly until the gland was irreversibly committed to involution. Since high levels of annexin-I are associated with the quiescent epithelial cell, annexin-I may play an important role in blocking differentiation of the mammary gland.