Data are presented here which verify that immature rats exhibit no circadian fluctuation of mitoses in the corneal epithelium prior to the third week of life. These data indicate that dexamethasone administration induces age-dependent alterations in the circadian mitotic pattern of the corneal epithelium. Nine day old animals still revealed no rhythmic component in circadian mitotic activity after dexamethasone treatment. Eleven-, 12- and 13-day old animals treated with dexamethasone exhibited circadian cycles which looked qualitatively like the circadian rhythm of the 15-day old animal. Fifteen-day old animals responded with a synchrony of mitotic activity at 0700 hr and a generalized decrease in mitotic activity. The possible dependence of these responses upon existing circadian mitotic activity and maturation of the central nervous system and/or the glucocorticoid target tissue is discussed.