The effects of RLX on the microvasculature of the mouse mammary gland are reported. RLX (pure porcine standard NIH-RXN-P1) at a dose of 3 GPU was administered subcutaneously to virgin adult mice ovariectomized 12 days before. The mammary glands were removed 18-20 h after RLX injection and their examination by light microscopy did not reveal any substantial growth-response to the hormone. Histology and morphometry indicated striking dilation of microvessels, especially capillaries, and electron microscopy revealed an increase in the micropinocytotic vesicles, thus suggesting enhanced transendothelial transport of substances. Such phenomena, which were independent of a release of granules by mast cells, may represent an important component of the mammotrophic action of RLX.