We have investigated the effect of the anterior pituitary hormone, prolactin, on mitosis in the lens epithelium of the galactose-fed rat. Our findings indicate that prolactin had a stimulatory effect on galactose-triggered mitosis within 24 hours after hormone administration. After three full days of galactose-feeding and daily prolactin injections, lens epithelia from prolactin treated rats had fewer mitoses than those from saline-injected controls. At the time when the number of mitoses had fallen to a subnormal level due to galactose-feeding (i.e., after 7 days), lenses of prolactin-treated animals exhibited less of a decrease in mitotic activity. Thus, it seems that prolactin modulates the mitotic response of lens epithelial cells when animals are on a galactose-rich diet. This modulation may be due, among other things, to a general acceleration of the cataractogenic process, mediated by the osmoregulatory action of prolactin.