Cultures of normal rat mammary cells have been developed which produce alpha-lactalbumin (a-LA) for several months in response to physiological levels of corticosterone whereas none is measureable without glucocorticoid. In contrast to recent studies in short-term organ cultures, these long-term cell cultures derived from virginal or mid-pregnant rats show no inhibition of a-LA production at high glucocorticoid concentrations. Comparable a-LA production was observed after a few months whether cells were initially cultured into media with various corticosterone levels or initially into high levels of corticosterone for 3 weeks prior to the shift to various lower doses.