The growth of the ventral and dorsolateral prostate, the coagulating glands and the seminal vesicles was studied morphologically in castrated rats following 21 daily injections of oestradiol benzoate or protamine zinc insulin alone or in combination. Oestradiol benzoate was given in daily doses of 0.010 mg, protamine zinc insulin in daily doses of 10 IU. In the ventral and dorsolateral prostate oestradiol had an androgen-like action inducing epithelial growth and secretory activity in the epithelium. In the seminal vesicles and coagulating glands oestradiol induced an increased fibromuscular growth. Protamine zinc insulin induced growth and secretory activity in the dorsolateral prostate, while no such effect was observed in the other accessory reproductive glands. Protamine zinc insulin accentuated the androgen-like action of oestradiol benzoate on the epithelium of the dorsolateral prostate but did not influence on the increased fibromuscular growth seen after oestradiol in the coagulating glands and the seminal vesicles.