Little information is available on the control of blood vessel growth. To obtain information on the factors that regulate the proliferation of capillaries, we have developed procedures to isolate skin endothelial cells from the marginal ear vein of the rabbit, and have studied the factors that affect endothelial cell growth in vitro. Using the growth and the incorporation of isotopes into rabbit skin endothelial cells in vitro, analysis of the effect of the proliferative and non-proliferative epidermis on skin endothelial cells has begun, and fractionation of intact epidermis to isolate those substances in skin that may alter endothelial cell proliferation has been initiated. The isolation of the Weibel-Palade body and its role in the endothelial cell physiology is under investigation.