A technique is described for the ultrasound-guided injection of 99mTc-macroaggregates into a splenic vein in order to quantitate portosystemic collateral circulation. The fraction of portal blood by-passing the liver was derived from the radioactivity trapped in the liver and lungs and was expressed as the shunt index (SI). The method was tested in healthy dogs without shunting, and in dogs with single hereditary portosystemic shunts before and one month after surgical closure of the shunt. The mean SI was 0.01 (range 0.01 to 0.05) in healthy dogs and 0.94 (0.69 to 1.00) in dogs with hereditary shunts. After partial closure of the shunts the SI decreased to 0.25 (0.03 to 0.70). There was a significant positive correlation between the logarithms of the concentration of ammonia in plasma and the SI (r = 0.87, P < 0.001).