Metabolic changes in rats fed a low protein diet were investigated during 3 weeks after weaning using lactalbumin (LP) as dietary protein source. The energy intake was higher and the weight gain lower in rats fed the low protein diet (6%, LP group) than in control rats (13% lactalbumin, C group). Low protein diet induced no changes in plasma glucose, free fatty acids, or triacylglycerol concentrations; however, plasma protein and urea concentrations were lower in LP than in C rats. Plasma free T3 was higher in LP than in C rats (+38%, day 21) and insulin progressively decreased during the experimental period (-56%, day 21) without change in glucagon. Liver glycogen and triacylglycerol concentrations (+40% and +180%, respectively, day 21), and cytosolic and mitochondrial redox states increased (+100% and +100%, day 21), and protein concentration was decreased (-15%, day 21). Pyruvate kinase (PK) and malic enzyme activities were higher in LP than in C rats throughout the experiment (+80% and +210%, respectively, day 21), and glucose-6-phosphate dehydrogenase (G6PDH) activity progressively decreased (-65%, day 21). Phosphoenolpyruvate carboxykinase (PEPCK) activity increased after 2 weeks on a LP diet (+35%, day 21) and fatty acid synthetase (FAS) activity increased only during the first week on the diet (+100%, day 7). Such hormonal and metabolic changes appeared to be associated with the development of a futile energy-wasting cycle between pyruvate and phosphoenolpyruvate.