The gut microfloral metabolism of sorbitol and lactulose was investigated in 10 healthy control subjects and 10 patients with cirrhosis of the liver, by measuring the pulmonary excretion of the fermentational gases hydrogen and methane during 3 h. A possible acute effect on venous plasma ammonia after a single dose of sorbitol or lactulose was also investigated. No significant difference in fermentation gas excretion was found after ingestion of sorbitol compared with lactulose in healthy controls or cirrhotic patients (p greater than 0.05). Neither sorbitol nor lactulose had significant effect on plasma ammonia concentrations during the first 3 h after ingestion. These findings indicate that both sorbitol and lactulose are extensively fermented by the colonic flora and that the possibility of replacing lactulose with the much cheaper sorbitol in the treatment of portal-systemic encephalopathy ought to be investigated. The suggested acute gut-ammonia trapping effect of lactulose caused by acidification of the gut lumen from fermentative end-products was not supported by the present findings.