PURPOSE: The inducible isoform of heme oxygenase(HO), HO-1, responds to hypoxia. HO-1 regulates vascular smooth muscle tone through carbon monoxide production. To investigate the possible role of HO-1 in low-flow priapism, we examined the expression and activity of HO-1 in artificially induced veno-occlusive priapism in rat. MATERIALS AND METHODS: Fourteen male Sprague Dawley rats were divided into 2 groups with 7 rats each. In the first group, low-flow priapism was induced using a vacuum-constriction device and a constriction rubber band; in the second group, low-flow priapism was induced using papaverine. We measured the expression level and activity of HO-1 in penile tissues after time periods of 0(control), 2, 3, 4, 8, 12, 24, and 48 hours. At the same time, the expression levels of i-NOS, e-NOS, and beta-actin(control) in penile tissues were also measured. RESULTS: In both groups, expression of HO-1 and HO-1 enzyme activities in penile tissue significantly increased in a time dependent fashion(p<0.01). However, there was no difference in the expression of i-NOS and e-NOS in both groups at any time period. CONCLUSIONS: HO-1 was induced over time in rats with artificially induced veno-occlusive priapism. Induction of HO-1 may play a protective role against hypoxic injury, but may also play an important role in the vicious cycle observed for low flow priapism. Increasing induction of HO-1 against hypoxic injury in a prolonged erectile state promotes sustained dilatation of corporal smooth muscle, and this may aggravate low-flow priapism.