Cortisol affects the growth of serially propogated, fibroblast cell cultures derived from the rabbit fetal lung in a manner which is dependent upon the gestational age of the material used: early in gestation (20 days), the hormone (10−7–10−5 M) stimulates [6-3H]thymidine incorporation into DNA, while in late gestation (28 days), cortisol (10−7 and 10−6 M) inhibits this process. Cultures derived from the rabbit fetal skin are inhibited by cortisol (10−5 M) at both gestational ages. Fibroblasts derived from lung, but not from skin, efficiently convert cortisone to cortisol and this activity increases with advancing gestation. Cortisol does not affect the incorporation of [3H]choline into lecithin by confluent cultures of any of the fibroblast types studied.