Twenty-four-hour-fasted rats were given fructose (4 g/kg) by gavage. Fructose absorption and the portal vein, aorta, and hepatic vein plasma fructose, glucose, lactate, and insulin concentrations as well as liver fructose and fructose 1-P, glucose, glucose 6-P, UDPglucose, lactate, pyruvate, ATP, ADP, AMP, inorganic phosphate (Pi), cAMP, and Mg2+, and glycogen synthase I and phosphorylase alpha were measured at 10, 20, 30, 40, 60 and 120 min after gavage. Liver and muscle glycogen and serum uric acid and triglycerides also were measured. Fifty-nine percent of the fructose was absorbed in 2 h. There were modest increases in plasma and hepatic fructose, glucose, and lactate and in plasma insulin. Concentrations in the portal vein, aorta, and hepatic vein plasma indicate rapid removal of fructose and lactate by the liver and a modest increase in production of glucose. The source of the increase in plasma lactate is uncertain. Hepatic glucose 6-P increased twofold; UDPglucose rose transiently and then decreased below the control level. Fructose 1-P increased linearly to a concentration of 3.3 mumol/g wet wt by 120 min. There was no change in ATP, ADP, AMP, cAMP, Pi, or Mg2+. Serum triglycerides and uric acid were unchanged. Glycogen synthase was activated by 20 min without a change in phosphorylase alpha. This occurred with a fructose dose that did not significantly increase either the liver glucose or fructose concentrations. Liver glycogen increased linearly after 20 min, and glycogen storage was equal in liver (38.4%) and muscle (36.5%).(ABSTRACT TRUNCATED AT 250 WORDS)