Castrated or juvenile monkeys were given estrogen, progesterone and relaxin in various doses and combinations. Relaxin induced: (1) a dilatation of the superficial endometrial blood vessels and proliferation of their endothelial cells; this effect may be regarded as due to relaxin alone; (2) an intensified differentiation of the endometrial stroma cells into predecidual cells and granulocytes, dependent on estrogen priming and the simultaneous injection of progesterone; (3) a periarteriolar mantle-like accumulation of granulocytes in the basal endometrium dependent on the above pretreatment; (4) a degranulation of the granulocytes and hypersegmentation of their nuclei following prolonged administration of relaxin with estrogen and progesterone. Morphologically and histochemically the granulocytes of the monkey are almost identical with those of the human uterus and with the granular cells in the decidua and mesometrial gland of the pregnant rat. Immunohistologically relaxin has been demonstrated in the granulocytes of man and rat. The changes in the arterioles brought about by exogenous relaxin occur under physiological conditions only with the increased formation of endogenous relaxin during early pregnancy. They have been described in the immediate vicinity of ovum implantation in man, monkey, and rat. Their function possibly lies in the preparation of blood spaces for nutrition of the young embryo.