This study investigated the metabolic changes with age in the Fischer 344 x Brown Norway rat and its suitability as an animal model of postmaturational insulin resistance. Specifically, we determined whether an age-associated decrease in glucose disposal is associated with diminished whole body insulin responsiveness and/or a decrease in glucose transporter (GLUT-4) protein and mRNA content in medial gastrocnemius muscle of male Fischer 344 x Brown Norway rats of ages 8, 18, and 28 months. Fasting plasma glucose was unchanged with age. There was a significant age effect on visceral adiposity, fasting plasma insulin levels, insulin responsiveness, and GLUT-4 protein content. Insulin responsiveness and GLUT-4 protein were lower in the 18-month-old rats than in the 8-month-old rats. The findings of age-associated increases in visceral adiposity and insulin resistance, and decreases in GLUT-4 in the Fisher 344 x Brown Norway rat, suggest that this rat strain may be an appropriate model for studying the effects of aging on glucose homeostasis.