Angiotensin II was applied daily in doses of 67 or 670 ng to a section of the chick embryo chorio-allantoic membrane from day 7 to day 14 after fertilization of the eggs. During this one-week period, it caused a significant, dose-dependent increase in the vascular density index. The increase obtained with 670 ng daily was comparable to that after daily administration of 1.7 micrograms adenosine, a known stimulator of angiogenesis. The data suggest a possible role for angiotensin II as a mediator of vascular growth.