The great majority of the rats were operated on when they were between 6 and 7 months of age that is, after thymus involution would normally have set in. It is highly probable that the suprarenal glands stand in closer relation to the sex organs of the females than in the male, and that removal of these glands disrupts the sex interrelationships between gonads, suprarenals, and thymus, this disturbance being more definitely expressed in the female than in the male. This difference in the suprarenals is independent of pregnancy. This weight difference becomes greater as the rat grows, and it appears at an early period of life indeed, it is obvious at about 35 days of age, in spite of the fact that sexual maturity is seldom attained before 60 to 90 days.

In the rat there is a definite sex difference in the weight of the suprarenals (20), the glands being much larger in the female than in the male. This difference, we believe, may shed some light on the mechanism of thymus enlargement following double suprarenalectomy. Our data, though inconclusive, would seem to indicate a sex difference, the secondary hyperplasia of the thymus being more marked in the female than in the male. The thymus enlarges even though the animal loses some weight after operation. The enlargement has been observed to follow bilateral ablation with such constancy that it may be said to occur in all rats which survive double suprarenalectomy in good condition for from 3 to 5 weeks. Experimental evidence has been presented showing that the thymus gland of the rat enlarges rapidly in those animals surviving double suprarenalectomy.

Suprarenal gland