After many generations of selective breeding, rats of the high-avoidance strain (SHA) average 67% avoidance responses in a two-way shuttle box, whereas those of the low-avoidance strain (SLA) average 0%. Adrenal gland weights, both absolutely and relative to body weight, are 40-50% greater in adult SLA than SHA rats of both sexes. Females of both strains have larger adrenal glands than males. Morphometry revealed that the difference in adrenal size in adults is entirely in the three cortical zones. The strain difference occurs as early as 21 days of age, whereas the sex difference appears only after puberty. A 2-min exposure to either vapor induced an elevation in adrenal concentration of corticosterone which was significantly greater in SHA than SLA animals of both sexes at some time periods following the ether stress. Despite having smaller glands, older previously stressed SHA rats have higher basal adrenal concentrations of corticosterone than do SLA animals. The reduced steroidogenesis in the large adrenals of SLA rats suggests that there may be an enzymatic defect of genetic origin in those animals.