Release patterns of the mitogen-induced cytokines IL-1 and IL2 by Long Evans rat unfractionated spleen cells, as well as expression of IL-2R on such cells at different postnatal days (neonatal to adult) were studied for female and male rats. IL-1 release was found to be low but in a more or less steady fashion in cells of both sexes over the entire period. However, IL-2 synthesis increased from an almost undetectable level up to a significant secretion at the time of weaning and continued to increase in female but not male spleen cells. IL-2 receptors were found to be expressed in moderate amounts already at early postnatal days and up to weaning. Thereafter, the synthesis of IL-2R followed a course essentially similar to that of IL-2, i.e. higher for female than for male cells. In the female rats, a hormonal influence was observed on the formation of IL-1 and IL-2 but not on the expression of IL-2R.