The effects of vitamin D3 and two analogues on embryonic angiogenesis were studied in 4.5-day-old chick embryo chorioallantoic membranes. The active metabolite of vitamin D3, 1 alpha,25-dihydroxyvitamin D3, and a synthetic vitamin D3 analogue, 22-oxa-1 alpha,25-dihydroxyvitamin D3, inhibited angiogenesis in a dose-dependent manner, the inhibition occurring in the picomolar range. In contrast, vitamin D3 was not effective. The results suggest that these two vitamin D3 analogues might be promising anti-angiogenic agents for controlling the angiogenesis which occurs in several pathological conditions, including tumor development.