Chronic hepatic encephalopathy is highly responsive to changes in diet, to antibiotic therapy and to ingestion of nondigestible disaccharides. The precise pathophysiology of chronic hepatic encephalopathy in individual cases is highly variable, although ammonia toxicity and production of neurotransmitterlike substances in the gut have been proposed to contribute to the overall syndrome of chronic hepatic encephalopathy. The support for this hypothesis is based on the empiric observation that reduction in protein intake, a catharsis or both are effective treatments for chronic hepatic encephalopathy. This study was performed to evaluate the effect of mild subclinical and low-grade (grade 0 to 1) chronic hepatic encephalopathy on gastric emptying and oral-cecal transit times. Thirty patients were studied. Ten had no evidence of chronic hepatic encephalopathy, as determined with a battery of neuropsychiatric studies (group 1); 10 had subclinical hepatic encephalopathy, as judged on the basis of abnormal neuropsychiatric test performance but normal neurological examination (group 2); and 10 had grade 1 hepatic encephalopathy. Each underwent a liquid gastric emptying study and a lactulose oral-cecal transit time study. No significant differences between groups were evident in the results of the gastric emptying studies. In contrast, the time required for a lactulose load to reach the cecum was significantly greater in the patients with hepatic encephalopathy (p < 0.01) and increased as a function of the hepatic encephalopathy grade. More interesting was that when those patients with hepatic encephalopathy were treated for 1 wk with oral neomycin (2 @/day) and reduction in protein intake to 40 @day, the oral-cecal transit time declined significantly to levels seen in subjects without clinical or neuropsychiatric evidence of chronic hepatic encephalopathy. These data suggest that (a) chronic hepatic encephalopathy is associated with prolongation of oral-cecal transit that cannot be attributed to delay in gastric emptying and (b) treatment of chronic hepatic encephalopathy with antibiotics and protein restriction reduces oral-cecal transit. These data suggest but do not prove that