OBJECTIVE The aim of this study was to develop a technique to measure collateral blood flow in portal hypertensive rats. METHODS Morphological techniques included inspection, casts and angiographies of portosystemic shunts. The main hemodynamic measurements were splenorenal shunt blood flow (transit time ultrasound method), percentage of portosystemic shunts and regional blood flows (microsphere method). In study 1, a model of esophageal varices was developed by ligating the splenorenal shunt. In study 2, morphological studies of the splenorenal shunt were performed in rats with portal vein ligation. In study 3, the relationship between splenorenal shunt blood flow with percentage of portosystemic shunts was evaluated in dimethylnitrosamine cirrhosis. In study 4, secondary biliary, CCl4 and dimethylnitrosamine cirrhosis were compared. In study 5, rats with portal vein ligation received acute administration of octreotide. In study 6, rats with dimethylnitrosamine cirrhosis received acute administration of vapreotide. RESULTS Blood flow of para-esophageal varices could not be measured. SRS blood flow was correlated with the mesenteric percentage of portosystemic shunts (r = 0.74, P < 0.05), splenic percentage of portosystemic shunts (r = 0.54, P < 0.05) and estimated portosystemic blood flow (r = 0.91, P < 0.01). Splenorenal shunt blood flow was 6 to 12 times higher in portal hypertensive rats, e.g., in portal vein ligated rats: 2.8 +/- 2.7 vs 0.3 +/- 0.1 mL.min-1 in sham rats (P < 0.01), and was similar in the different cirrhosis models but was higher in portal vein ligated rats than in cirrhotic rats (1.2 +/- 0.7 vs 0.6 +/- 0.6 mL.min-1.100 g-1, P = 0.05). Octreotide significantly decreased splenorenal shunt blood flow: -23 +/- 20% (P < 0.01) vs -6 +/- 8% (not significant) in placebo rats. The variation of splenorenal shunt blood flow after vapreotide was significant but not that of the splenic percentage of portosystemic shunts compared to placebo. CONCLUSIONS The splenorenal shunt is the main portosystemic shunt in rats. The measurement of splenorenal shunt blood flow is easy, accurate and reproducible and should replace the traditional measurement of the percentage of portosystemic shunts in pharmacological studies.