A bioassay cascade superfusion procedure was used to compare and contrast the actions of arterial and venous endothelium-derived relaxing factor (EDRF) with authentic nitric oxide (NO) on several nonvascular smooth muscle preparations. EDRF was released from human umbilical vein or bovine pulmonary artery by A23187 and allowed to superfuse two nonvascular and one vascular precontracted smooth muscle strips arranged in a cascade. NO or S-nitroso-N-acetylpenicillamine was delivered by superfusion. Both EDRF and NO relaxed bovine trachea, although artery was 10 times more sensitive than trachea to either relaxant. Similarly, rabbit taenia coli and rat fundus relaxed in response to high concentrations of NO or large amounts of EDRF released from umbilical vein. Vascular and nonvascular relaxant responses to both EDRF and NO were inhibited by oxyhemoglobin, methylene blue or superoxide, and were enhanced by superoxide dismutase. Perfusion of pulmonary artery or umbilical vein with A23187 resulted in contraction of guinea pig ileum and relaxation of pulmonary artery, whereas NO relaxed both preparations. Oxyhemoglobin enhanced the contractile and abolished the relaxant responses. Thus, ileum is more sensitive to endothelium-derived contracting factor(s) than to EDRF. NO raised cyclic GMP levels in all smooth muscle preparations, but a greater fold increase was observed in artery than in nonvascular smooth muscle. EDRF released from human umbilical vein was identified chemically as NO or a nitroso compound, as was done previously for EDRF from bovine pulmonary artery and vein. These observations support the view that one EDRF from artery and vein is NO or a labile nitroso compound.