We have character-ized the ability of several cell types associated with the microvasculature of solid tumors to release nitric oxide (NO·) in response to increases in cytosolic Ca2+ concentration ([Ca2+]c). EA.hy926 immortalized human umbilical vein endothelial cells (EC), rat fibroblasts (RFL), and tumorigenic cells isolated from R3230Ac rat mammary adenocarcinoma (MaC) were treated with thapsigargin (TG), an inhibitor of Ca2+ATPase. NO· output was measured via a chemiluminescence detection system. Baseline NO· output was detectable only for EC. TG caused a significant increase in EC NO· output that could be blocked with NG-monomethyl-L-arginine and restored with L-arginine. TG did not stimulate NO· release from RFL or MaC cells, despite elevating [Ca2+]c in all cells. A Ca2+-dependent isoform of NO synthase (eNOS) was detected by immunoblot only in EC. These data indicate that EC, but not RFL or MaC, are capable of Ca2+-dependent NO· release and suggest that any Ca2+-dependent NO· release within this tumor is primarily of endothelial (and not tumorigenic cell) origin. Copyright ©1996 the American Physiological Society.