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The value of performing arterial portography during reactive hyperemia was investigated in four dogs with presinusoidal cirrhosis, stable portal hypertension in excess of 20 cm of water, and extensive porto-systemic venous collaterals, and compared to tolazoline (1 mg/kg) and control studies. With SMA balloon occlusion the maximum decrease in portal flow and pressure occurred between 1 and 2 minutes. During reactive hyperemia following immediately the release of a 2-minute SMA occlusion, portal flow and pressure increased from pre-occlusion values (mean ± 1 SE, n:4) of 15 ± 2 ml per min per kg and 25 ± 1 cm H2O to 32 ± 5 ml per min per kg and 40 ± 2 cm H2O. With reactive hyperemia both significant higher peak iodine concentrations in the portal blood and significant improvement in visualization of the portal system and porto-systemic venous collaterals occurred when compared to tolazoline or control angiograms. With reactive hyperemia both peak blood iodine concentrations and maximum opacification of the portal vein occur 2 to 3 and 4 to 6 seconds earlier than with tolazoline or in controls, respectively. Compared to controls, tolazoline increased peak iodine concentrations in portal vein significantly and improved visualization of the portal system and collaterals in 7 of 12 examinations. Judged from the experience in peripheral arteriography, performance of angiography during reactive hyperemia appears to be a very safe procedure. However, the use of balloon catheters carries additional risks particularly when not properly used.