The [14C]aminopyrine breath test (ABT) was used before and after surgical portocaval shunting in 29 cirrhotic patients, 15 with arterialization of the portal vein and 14 without. These two subgroups were comparable with regard to age, prothrombin time, serum albumin, serum total bilirubin concentrations, Child classification, and preoperative ABT. After portocaval shunt, a striking reduction in ABT values was observed after 6 and 12 months (90.7 +/- 8.0%, P < 0.05 and 77.3 +/- 7.5%, P < 0.005, respectively). No further decrease was observed between 12 and 24 months. The microsomal function was reduced to a greater extent in patients with postnecrotic cirrhosis (55 +/- 4.8%, P < 0.01) than in those with alcoholic cirrhosis (84.8 +/- 10.4%, P < 0.05). Arterialization of the portal stump of the portal vein did not prevent the reduction in hepatocellular function. No significant change in prothrombin time, serum albumin, or bilirubin concentrations was observed during the follow-up period in any of the groups. Overall, preoperative ABT was significantly higher in those surviving one year (0.29 +/- 0.12% administered dose) than in those who died within this time period (0.12 +/- 0.01%, P < 0.005). This observation suggests that portocaval anastomosis induces an early decrease in microsomal function that is not prevented by arterialization of the portal vein. It also suggests that ABT might be helpful in selecting patients for shunt surgery.