Concentrations of immunoreactive insulin and of glucose were measured in plasma samples taken at 4-hour intervals from cirrhotic and normal control subjects ingesting diets containing various amounts of protein. In both groups, insulin levels were highest after meals, between 11 a.m. and 7 p.m.; in cirrhotic subjects, but not in controls, plasma glucose levels were also higher at these times. Insulin and glucose concentrations in plasma from cirrhotic subjects were higher than those from control subjects at all times measured. Peak daytime insulin levels were about 150 muU/ml in plasma from cirrhotic subjects, and 40--50 muU/ml in plasma from controls; nocturnal levels were 20--30 muU/ml in cirrhotics versus 10 muU/ml or less in controls. The protein content of the diet was unrelated to plasma insulin levels at all times of day or night examined. These data thus show that plasma insulin levels are persistently elevated in cirrhotics, remaining higher than normal throughout the 24-hour period.