The pharmacokinetics of Naproxen administered as a single oral dose of 250 mg, have been determined in 7 healthy volunteers and 9 patients who had been diagnosed as suffering from diabetes mellitus with varying degrees of angiopathy. A two-compartment model was used to describe the biphasic decline in serum concentrations and to calculate the amount of drug in the central and peripheral compartments. In healthy volunteers the following values were obtained for various pharmacokinetic parameters: tmax = 2 hr; Cmax = 52.63 micrograms/ml; Ka = 1.893 hr-1; alpha = 0.393 hr-1; beta = 0.049 hr-1; K12 = 0.147 hr-1; K21 = 0.198 hr-1; K13 = 0.097 hr-1. In patients with severe diabetic microangiopathy, a decrease may be seen in the fraction of the dose absorbed shown by a decrease in the Cmax and the (AUC) 0--8 hr. The glomerular impairment of some patients leads to a decrease in the elimination constant.