The aim of this study is to investigate the effects of nitric oxide, formed from L-arginine, on the production of endothelin?1 in vivo and in cultured endothelial cells. In mechanically ventilated anesthetized dogs (n = 5), mean pulmonary arterial pressure (PAPm) and pulmonary vascular resistance (PVR) during hypoxic ventilation (FIO2 = 0.10) was 25 ?3.1 kPa and 68.7 ?10.2 kPa.s / L respectively. IG-nitro-L-arginine methylester (L-NAME), an inhibitor of nitric oxide synthase, increased the peak value of PAPm and PVR during hypoxic ventilation to 36.6 ?4.7 kPa and 158.4 ?25 kPa.s / L and its effect lasted for 2-3 hours. Meanwhile, plasma endothelin? level in the femoral artery increased by 20.9+ 7.1, 25.6?7.7, 28.6?7.9 pg / ml at the 60 th, 120th, 180th minute after the injection of L-NAME respectively (P<0.05 vs hypoxic control before the injection). In cultured endothelial cells from umbilical veins, endothelin-1 level of culture medium in control group was 35.1 ?.9 pg / 105 cells /ml (n=9). L-NAME inc The aim of this study is to investigate the effects of nitric oxide, formed from L-arginine, on the production of endothelin? 1 in vivo and in cultured endothelial cells. (PAP) and pulmonary vascular resistance (PVR) during hypoxic ventilation (FIO2 = 0.10) was 25? 3.1 kPa and 68.7? 10.2 kPa.s / L respectively. IG-nitro-L-arginine methylester of nitric oxide synthase, increased the peak value of PAPm and PVR during hypoxic ventilation to 36.6-4.7 kPa and 158.4-25 kPa.s / L and its effect lasted for 2-3 hours. Meanwhile, plasma endothelin? level in the femoral artery increased by 20.9+ 7.1, 25.6-7.76, 28.6- 7.9 pg / ml at the 60th, 120th, 180th minute after the injection of L-NAME respectively (P <0.05 vs hypoxic control before the injection). In cultured endothelial cells from umbilical veins, endothelin-1 level of culture medium in control group was 35.1? .9 pg / 105 cells / ml (n = 9). L-NAME inc