The development of sex differences in the adrenal morphology and responsiveness in stress of rats from birth to the end of life.

The appearance and development of sex difference in the adrenal cortex of rats have been studied. Morphological and secretory differences between the adrenal cortex of female and male rats begin on the 40th day of postnatal life, when females respond in stress by a greater increase in plasma corticosterone concentration. The sex difference becomes fully manifest at the age of 55 days (females have heavier absolute and relative adrenal glands and respond to stressful stimuli by a greater increase in adrenal and plasma corticosterone concentrations). At the age of 11 to 23 months the adrenal corticosterone concentrations in stress are equal in both sexes, and the absolute adrenal weights are similar from 18.5 months until death. The body weight gain is equal in both sexes during the first 50 days of life and greater in males from day 50 to 1 year of age. After this age the body weight of males remains almost the same (315 to 322 g)8 while females continue gaining weight until the end of life (196 to 231 g). This possible causal relationship between the rhythm of growth and sex difference in the adrenal glands in rats is discussed.