Postprandial storage of carbohydrate as glycogen in muscle was quantitated in normal subjects (n = 8) by natural abundance 13C-nuclear magnetic resonance spectroscopy with proton decoupling in a 4.7-tesla magnet. After an overnight fast three basal measurements of gastrocnemius muscle glycogen were made and a mixed meal was given. Muscle glycogen concentration rose from 83.3 +/- 5.2 to a maximum of 100.2 +/- 6.7 mmol/l muscle at 4.9 h (P < 0.01) and fell thereafter to 90.6 +/- 5.9 mmol/l muscle at 7 h postprandially (P < 0.006). The meal brought about an increase in plasma glucose from 5.4 +/- 0.2 to 7.3 +/- 0.4 mmol/l at 30 min but this was followed by a rapid fall to 6.2 +/- 0.4 mmol/l at 75 min. Plasma insulin rose from 62.4 +/- 11.4 to 900 +/- 216 pmol/l at 30 min and declined steadily thereafter. It was calculated from total muscle mass measurements and estimation of carbohydrate absorption rates that at peak muscle glycogen concentrations between 26 and 35% of the absorbed carbohydrate was stored as muscle glycogen. These data quantitate the role of skeletal muscle glycogen synthesis in postprandial carbohydrate storage and demonstrate that this tissue acts as a dynamic buffer to maintain glucose homeostasis during postprandial substrate storage.