In this paper data resulting from an investigation into the influence of sex on selected circadian rhythms in lymphoreticular organs, liver, kidney, and corneal epithelium of adult CD2F1 mice are reported. Increased organ weight, total DNA and RNA in the spleen, total DNA and [3H thymidine [( 3H]TdR) incorporation into DNA in the thymus, and [3H]TdR incorporation into DNA in the liver and bone marrow in female mice compared to male littermates is demonstrated. In contrast, kidney weight and [3H]TdR incorporation into DNA as well as the corneal epithelium mitotic index are greater in male mice. Except for the corneal epithelium mitotic index and total splenic RNA and DNA, circadian rhythmicity in the variables studied is validated using the cosinor method of rhythmometric analysis in male but not in female mice. The lack of sinusoidal rhythmicity in female mice is presumed to be due to asynchrony of estrous cycling between mice within this group. Moreover, a differential organ response to exogenous testosterone enthanate is reported. The administration of this hormone suppresses [3H]TdR incorporation into DNA in the thymus and liver but not in the spleen or bone marrow at 18, 42, or 72 hr after injection.