OBJECTIVE: We investigated if alterations in the insulin-signaling pathway could contribute to reduced hepatic glycogen levels in adult rats subjected to a protein deficiency during intrauterine life and lactation and reared through to recovery on a soybean diet. METHODS: Rats from mothers fed with 17% or 6% protein (casein) during pregnancy and lactation were maintained with a 17% casein diet (offspring born to and suckled by mothers fed a control diet and subsequently fed the same diet after weaning [CC group] and offspring born to and suckled by mothers fed a control diet and subsequently fed a soybean flour diet with 17% protein after weaning [CS group]), a soybean diet (offspring of mothers fed a low-protein diet and a control diet after weaning [LC group] and offspring of mothers fed a low-protein diet and fed a soybean flour diet containing 17% protein after weaning [LS group]), or a 6% casein diet (offspring of mothers fed a low-protein diet and subsequently fed the same diet after weaning [LL group]) from weaning until 90 d of life. RESULTS: A soybean diet did not modify basal serum glucose and glucagon concentrations, but raised basal serum insulin and consequently increased the serum insulin/glucose ratio. Insulin receptor and insulin receptor substrate-1 levels were lower in rats fed a soybean diet compared with those maintained with a casein diet. In the LS group, the p85 levels were higher than in the LC group, whereas in CS rats its expression was lower than in CC rats. The expression of p110 was lower in the CS group compared with the CC group and similar in the LS and LC groups. Insulin receptor substrate-1 phosphorylation was similar in the LS, LC, and CS groups and lower compared with the CC group. The insulin receptor substrate-1-p85/phosphatidylinositol 3-kinase association was lower in LS than in LC rats and in CS than in CC rats. Akt phosphorylation was lower in the CS and LS groups than in the CC and LC groups. CONCLUSION: Adult rats maintained with a soybean diet exhibited insulin resistance due, at least in part, to alterations in the early steps of the insulin signal transduction pathway.