Cultured bovine endothelial cells were seeded onto the intimal surface of endothelium-denuded rings of canine coronary artery. These rings did not previously relax to acetylcholine, substance P, bradykinin, and A23187. After seeding, the same rings relaxed to bradykinin and A23187, but not to acetycholine or substance P. Indomethacin pretreatment did not affect these responses. Cells from the same source were then grown to confluence on microcarrier beads, poured into small columns, and perfused with Krebs+ solution. The perfusate from the columns was bioassayed on endothelium-denuded rings of coronary artery from either the dog or pig. Challenge of the column in the presence of indomethacin with either bradykinin or A23187 as well as acetylcholine or substance P caused release of a substance that relaxed both types of artery. Its activity half-life was 6.4 ± 0.4 sec at 37°C and it was hydrophilic and negatively charged. Prostacyclin (PGI2) as a candidate for EDRF was ruled out because (1) indomethacin failed to block its release and (2) the pig coronary artery, although insensitive to PGI2, relaxed to the endothelium-derived substance. These results show that, in response to a number of dilator drugs, cultured endothelial cells release a vascular relaxing substance (EDRF) that has characteristics similar to the EDRF of normal endothelium. The chemical nature of EDRF awaits clarification.