The aim of the present study was to determine the effects of feeding various dietary proteins on insulin sensitivity and glucose tolerance in rats. Male Wistar rats were fed for 28 days with isoenergetic diets containing either casein, soy protein, or cod protein. Cod protein-fed and soy protein-fed rats had lower fasting plasma glucose and insulin concentrations compared with casein-fed animals. After intravenous glucose bolus, cod protein- and soy protein-fed rats induced lower incremental areas under glucose curves compared with casein-fed animals. Improved peripheral insulin sensitivity was confirmed by higher glucose disposal rates in cod protein- and soy protein-fed rats (15.2 +/- 0.3 and 13.9 +/- 0.6 mg. kg(-1). min(-1), respectively) compared with casein-fed animals (6.5 +/- 0.7 mg. kg(-1). min(-1), P < 0.05). Moreover, test meal experiments revealed that, in the postprandial state, the lower plasma insulin concentrations in cod protein- and soy protein-fed animals could be also due to decreased pancreatic insulin release and increased hepatic insulin removal. In conclusion, the metabolic responses to three common dietary proteins indicate that cod and soy proteins, when compared with casein, improve fasting glucose tolerance and peripheral insulin sensitivity in rats.