The adrenal gland is of critical importance for a plethora of biological processes. We performed the first systematic analysis of adrenal gland growth using unbiased stereological methods in male and female mice from weaning to adulthood ( weeks 3, 5, 7, 9, and 11) at the organ, compartment, and cellular levels. Adrenal weights increased from week 3 to week 7 in male and female mice, remained at this level in females, but decreased by 25% between week 7 and week 9 in males. Female adrenal glands displayed a higher weight at any stage investigated. The volume of the zona fasciculata was consistently higher in female vs. male mice. In both genders, the number of zona fasciculata cells reached a maximum at the age of 7 wk and decreased significantly until week 9. Serum corticosterone concentrations decreased from 3 to 11 wk of age both in male and female mice. However, the estimated total amounts of corticosterone in the circulation were similar in 3- and 11-wk-old mice. Furthermore, total circulating corticosterone was higher in females than in males at an age of 5 and 11 wk. In the zona glomerulosa and in the X-zone, time- and gender-dependent growth effects were observed. In conclusion, our results demonstrate that growth and function of the adrenal glands are markedly influenced by gender and age. These factors require careful consideration in studies aiming at the functional dissection of genetic and environmental factors affecting adrenal growth and function.