Weights were obtained and histological examinations made of the adrenal glands and reproductive organs of 193 female Microtus pennsylvanicus trapped on the grasslands of the Letterkenny Army Ordnance Depot in south central Pennsylvania. The relationships between adrenal weight and histology and reproductive status and population size were examined. Adrenal weight increased sharply at sexual maturity but there were no further changes in adrenal weight relative to body weight with changes in reproductive status once maturity was reached. It also was concluded that the inner juxtamedullary portion of the adrenal cortex of female voles is inner-fasciculata-reticularis and not an X-zone and that the adrenal hypertrophy at maturation is probably due to increased secretion of estrogens. There was marked and significant parallelism between mean adrenal weights and population size for mature females; but less so for immature females. Changes in adrenal weight, particularly in the mature females, were attributed to changes in population size and not to reproductive status or directly to seasonal environmental changes.