SUMMARY: The relationship of structure and function in the adrenal gland of the possum Trichosurus vulpecula, has been studied using in-vitro incubation techniques. It was shown that both 17α-hydroxycorticosteroids and 17-deoxycorticosteroids were produced from radioactive pregnenolone and progesterone, and that these transformations occurred both in the definitive cortex as well as in a special zone of hypertrophied tissue found only in the adult female. In support of earlier findings, it was also shown that the adrenal cortex of the possum has a remarkable capacity to produce C19 steroids (including androstenedione and testosterone) from the radioactive precursors. While most of the transformations occurred with equal efficiency in both types of tissue, the reduction of androstenedione to testosterone seemed to take place more readily in the special hypertrophied zone of the adult female. In studies in vivo, it was found that levels of testosterone in the peripheral blood of the adult female possum were extremely high compared with man. Variations in testosterone levels were not apparently correlated with the stage of the oestrous cycle. The possible pathways by which the adrenal products are synthesized, and their physiological implications are discussed.