Angiogenic capability of PGE2 [prostaglandin E2] was tested by implanting pellets of an ethylene vinyl acetate slow release polymer containing PGE2 on the chorioallantoic membrane of 8-day-old chicken embyros. Elvax pellets releasing approximately 0.2, 2.0 or 20 ng/day PGE2 induced neovascular responses. Pellets releasing 2.0 or 20 ng/day of PGA2, PGF2, or TXB2 [thromboxane B-2] did not appear to be angiogenic when compared with PGE2. These release rates of PGE2 are similar to those reported for a variety of tumors, activated macrophages, inflammatory exudates and rheumatoid synovia, suggesting that PGE2 may be a key factor in various neovascular reactions.