The key point in the diagnostic algorithm for pulmonary hypertension plays the pulmonary reactivity testing, the agent most frequently used in the test is inhaled nitric oxide (iNO). Various dosages of iNO were used in acute vasoreactivity testing so far, so we aimed to determine the most effective dose of iNO administered in the acute vasoreactivity testing. Ten consecutive patients was enrolled to the open label study. To assess the most effective dose of iNO increasing concentrations of iNO: 10 ppm, 20 ppm and 30 ppm were administered. In the study statistically significant reduction in mean pulmonary artery pressure after each dose of iNO as compared to baseline was found. There were no significant differences in mean pulmonary artery pressure between subsequent iNO doses (10 ppm vs. 20 ppm, 20 ppm vs. 30 ppm). Statistically significant reduction of systolic pulmonary artery pressure after iNO at the dose of 20 ppm as compared to a dose of 10 ppm was observed. No significant side effects during iNO administration were observed. We concluded that the dose of inhaled nitric oxide used in the acute vasoreactivity testing should not exceed 20 ppm, it is effective and safe dose.