In a cross-sectional study we have examined the regulatory effect of insulin in vivo on the major insulin regulatable glucose transporter (GLUT4) in vastus lateralis muscle from 12 noninsulin-dependent diabetes mellitus (NIDDM) patients and 8 healthy control subjects. Insulin-stimulated glucose uptake rate in peripheral tissue was decreased by 41% (P < 0.01) in NIDDM patients compared to healthy subjects, whereas no significant differences could be shown in the abundance of total GLUT4 protein per DNA or GLUT4 messenger RNA (mRNA) per DNA among the 2 groups in muscle biopsies obtained in the basal state. In healthy subjects, 4 h of insulin infusion (2 mU/kg/min) induced a 31% reduction (P < 0.05) in the total GLUT4 protein content per DNA and a 35% increase (P < 0.05) in GLUT4 mRNA per DNA, whereas the GLUT4 mRNA and protein responses to insulin were heterogenous and statistically unaltered in the NIDDM patients. The GLUT4 protein per DNA of muscle obtained in the basal state correlated positively with the in vivo insulin-stimulated glucose uptake rate in the control group (r = 0.82, P < 0.05), whereas there was no comparable correlation in the NIDDM group (r = 0.05, P = 0.88). Furthermore, GLUT4 protein content in skeletal muscle after 4 h of insulin infusion did not correlate with insulin-stimulated glucose uptake in any of the groups. In conclusion, 4 h of insulin infusion causing supraphysiological serum insulin levels modulates the expression of GLUT4 in skeletal muscle from healthy subjects, with divergent effects at protein and mRNA levels. The physiological significance of these observations will have to be elucidated in future studies. Factors other than total GLUT4 protein content of muscle play a role in determining insulin-stimulated glucose uptake in human skeletal muscle.