The effects of radiation on the release of mitogenic factors into the media of cultured endothelial cells of bovine, porcine, and human origins were studied. Although unirradiated controls revealed a significant background activity, single doses of irradiation (20-60 Gy) resulted in a dose-related increased release of growth factor activity, measured by the mitogenic effects of the conditioned media on both 3T3 mouse fibroblasts and unirradiated endothelial cells serving as target cells. Receptor binding competition assays for the platelet-derived growth factor receptor revealed that 12-28% of the total mitogenic activity was due to platelet-derived growth factor-like mitogens. Mitogenic assays using endothelial cells and specific antibody mediated inhibition assays suggested that another component of the mitogenic activity was due to a fibroblast growth factor-like factor. Although radiation resulted in a significant increase in cell death, the enhanced growth factor activities did not appear to result from cell lysis-related leakage of intracellular stores of growth factor. Instead, our data suggest that the growth factors were synthesized de novo and secreted at elevated levels by the cells which maintained postradiation a high level of metabolic activity. Time course studies demonstrated that the growth factors accumulation in the conditioned media started within the first 24 h after radiation and reached a plateau within 72 h after treatment. Radiation-induced release of endothelial cell-derived growth factors may be involved in the pathogenesis of both early vascular damage and the late fibrosis which represents a prominent feature of late radiation damage in normal tissues.