Plasma glucose and insulin concentration following a 75 g oral glucose challenge and glucose uptake during a hyperinsulinaemic glucose clamp study were determined in 50 non-obese individuals. The study population was divided into five groups on the basis of their glucose tolerance: normal, impaired glucose tolerance, Type 2 (non-insulin-dependent) diabetes mellitus with fasting plasma glucose of less than 8 mmol/l, between 8-15 mmol/l, and more than 15 mmol/l. The plasma insulin response was significantly greater (p less than 0.001) than normal in those with either impaired glucose tolerance or Type 2 diabetes and a fasting plasma glucose concentration less than 8 mmol/l.(ABSTRACT TRUNCATED AT 250 WORDS)