Methylene blue (MB) is a soluble guanylate cyclase inhibitor, and known as an endothelium-derived relaxing factor (EDRF) inhibitor in vitro. In the present study, it was demonstrated that intravenous administration of MB caused a dose-dependent hypertensive effect in rats. The hypertensive responses to the higher doses (10 and 20 mg/kg) of MB was followed by a reflex hypotension which did not appear in pithed rats. Noradrenaline depletion by reserpine pretreatment did not inhibit MB-induced hypertension, but abolished the hypotensive response. Both hypertensive and hypotensive phases were not altered by indometacin. These results may suggest that in vivo guanylate cyclase inhibition leads to an increase in blood pressure; prostaglandins and noradrenaline release from sympathetic nerve endings do not contribute to MB-induced hypertension and it may be due in part to the inhibition of EDRF.