The impact of the dynamic changes in plasma glucose and insulin levels observed during a frequently sampled intravenous (IV) glucose tolerance test (FSIGT) on whole-body glucose processing and muscle glycogen metabolism is not known. Paired randomized FSIGTs were performed in eight healthy subjects (age, 31 years; range, 28 to 35; BMI, 25.4 kg/m2; range, 22.3 to 32.1), one with muscle biopsy samples and one without. The mean time average (0- to 40- and 0- to 120-minute) insulin levels during the test were 26.6 and 11.4 mU/1, respectively. Glucose oxidation increased following the IV glucose bolus (basal 1.34 +/- 0.21 v mean value at 0 to 120 minutes 2.09 +/- 0.22 mg/kg fat-free mass [FFM]/min, P < .02). In contrast, fractional glucose-6-phosphate [G-6-P]) (0.1/10 mmol/L) skeletal muscle glycogen synthase activity in muscle biopsies obtained before and following the IV glucose bolus (-30, 30,60, and 120 minutes, respectively) were unchanged (38.1% +/- 2.3%, 38.3% +/- 2.9%, 38.1% +/- 2.3%, 35.4% +/- 2.3%, NS). Skeletal muscle glycogen concentration decreased slightly (449 +/- 54, 439 +/- 55, and 383 +/- 29, and 438 +/- 48 mmol/kg dry weight, P =.05), indicating no net storage of glucose into glycogen during the FSIGT. G-6-P decreased (0.77 +/- 0.08, 0.64 +/- 0.07, 0.66 +/- 0.07, and 0.54 +/- 0.04 mmol/kg dry weight, P < .05). Levels of the insulin-regulatable glucose transporter, GLUT-4, were unchanged. Insulin sensitivity (Si), glucose effectiveness, and insulin secretion parameters (01 and 02) were not affected by the muscle biopsy procedure. In conclusion, the FSIGT is associated predominantly with increased whole-body glucose oxidation with no apparent activation of muscle glucose storage as glycogen. Thus, the Si measured by the FSIGT, although similar in magnitude to the clamp-derived parameter, represents primarily glucose oxidation, in contrast to the euglycemic clamp, which involves glucose oxidation and storage.