Normal volunteers were infused for 10 days with high (Group I, n = 2, ca 700 g/day) or low (Group II, n = 3, ca 150 g/day) glucose loads to evaluate nitrogen metabolism and balance. Nitrogen excretion was least in Group I subjects, but both groups lost less nitrogen than a comparable group of totally fasted normal volunteers. Serum insulin levels were higher in Group I subjects while plasma glucose levels did not change significantly in either group. Blood pyruvate levels doubled and triglyceride levels increased, while free fatty acid levels declined in Group I subjects. In contrast, pyruvate and triglyceride levels did not change, while free fatty acid levels rose in Group II subjects. Plasma amino acid analysis revealed a marked reduction of branched chain amino acids, together with a marked increase in alanine levels in Group I subjects. A more modest decline in branched chain amino acids, together with a decline in alanine levels, were seen in Group II. Thus, in Group I, the higher insulin levels elicited by the larger glucose load inhibited lipolysis, the release of amino acids by peripheral tissues, and presumably hepatic gluconeogenesis. In Group II subjects, in whom lower insulin levels were elicited, lipolysis was not inhibited, as evidenced by an increase in free fatty acid levels. In addition, apparent inhibition of amino acid release by peripheral tissues as well as hepatic gluconeogenesis were less pronounced in these subjects compared with those in Group I.