After diet stabilization insulin action was measured at plasma insulin levels of 100MuU/ml and 250MuU/ml using the euglycemic clamp technique, in thirten obese noninsulin dependent diabetics (NIDDM). Fasting and two hour blood glucose concentrations were then normalized for a period of one month using insulin. After this period insulin action was reassessed similarly in all subjects and then reassessed a further two weeks after therapy withdrawal. The results show that normalization of blood glucose over one month improves insulin action at plasma insulin concentrations of 100MuU/ml (P less than 0.05) and 250MuU/ml (P less than 0.0003). Further this improvement is maintained for at least weeks after therapy withdrawal. Thus it appears that hyperglycemia contributes to the impaired insulin action of NIDDM, and that insulin action can be improved by normalization of blood glucose, without weight loss, in NIDDM.