We examined the possible mechanisms for carbon monoxide (CO)-induced effects in hearts isolated from Wistar rats exposed for 1 wk to 530 ppm CO. They were treated by daily intraperitoneal injections of either methylene blue (10 mg/kg), glibenclamide (3 mg/kg), or apamin (125 nmol/kg), known to inhibit vasodilatory mechanisms. Hearts were excised, cannulated, and retrogradely perfused through the coronary artery, using the Langendorff method with a constant perfusion pressure. After stabilization, we investigated the degree of resistance to an in vitro transient global low-flow ischemia. One-week CO exposure induced a significant increase in hematocrit and a cardiomegaly, which were not affected by any of these drugs. An enhancement of coronary flow was observed concomitantly with a decrease in ischemic contracture after CO exposure. However, no improvement of contractile recovery was noted. Since methylene blue did not interact with the CO effects, a cGMP signaling pathway can be excluded. On the contrary, as glibenclamide and to a greater extend apamin blocked the vasodilatory effects of CO, we conclude that K+ channels may be involved in these CO effects.