Abstract 1. 1. Adrenal size and activity in reproductively inhibited young born into laboratory populations of the white-footed mouse were examined. Measurements were made of body weight, reproductive organ weight and adrenal weight. Serum corticosterone was measured by radioimmunoassay. 2. 2. Data from reproductively inhibited animals were compared with corresponding values from reproductively capable animals of the same sex. The mean paired testis and seminal vesicle, or paired ovary and uterus, weights, were significantly reduced in reproductively inhibited animals of both sexes. The paired adrenal weights were not different in any comparison. 3. 3. Reproductively inhibited males selected from laboratory populations had a mean corticosterone concentration that was significantly higher than the corresponding value for reproductively capable males. Females showed no significant difference in mean serum corticosterone concentrations. 4. 4. The results are discussed in relation to earlier studies in two species of Peromyscus and the apparent paradox of elevated serum corticosterone in the absence of adrenal hypertrophy.