Rat aorta was homogenized and the 13000 x g supernatant fraction was tested for heme oxygenase (HO) activity by using a sensitive gas chromatographic method to measure carbon monoxide (CO), one of the products of the HO reaction. The rate of NADPH-dependent CO formation, an index of HO activity, was 1.41 +/- 0.40 nmol CO.mg(-1)protein. h(-1) in the rat aorta supernatant fraction and 2.05 +/- 0.55 nmol CO.mg(-1) protein.h(-1) in the rat liver 13000 x g supernatant fraction, a tissue known to contain HO activity. Chromium mesoporphyrin (0.05 mM), an inhibitor of rat liver HO, significantly decreased HO activity by 26% in the aorta supernatant fraction and 50% in the liver supernatant fraction. On the basis of the results of this study, which demonstrated HO-catalyzed CO formation in aortic tissue, and previous observations that CO relaxes vascular smooth muscle, we suggest that a physiological role for CO in vascular smooth muscle relaxation should be further investigated.