1. The concentrations of adenosine 3′:5′-cyclic monophosphate (cyclic AMP) in human adipose tissue were studied after intravenous insulin administration (0·3 unit/kg body weight) in obese diabetic and obese non-diabetic subjects.
2. The declines in plasma glucose, glycerol and free fatty acids in the two groups after insulin were similar if allowances were made for the fasting hyperglycaemia in the diabetic subjects.
3. Insulin in vivo lowered the concentrations of cyclic AMP in adipose tissue from non-diabetic subjects.
4. Insulin administration in vivo did not alter the tissue concentrations of cyclic AMP in adipose tissue of obese diabetic subjects. This lack of effect of insulin provides evidence for tissue resistance in vivo to the action of insulin in diabetes mellitus.