A simple and rapid colorimetric method for the assay of erythrocyte and plasma glutathione reductase (GR) activity is described. The method is based on the colorimetric measurement of reduced glutathione (GSH) (1) produced when the enzyme is incubated with oxidised glutathione (GSSG) in the presence of either reduced nicotinamide adenine dinucleotide (NADH) or reduced nicotinamide adenine dinucleotide phosphate (NADPH). Results of investigations on the effects of substrate and coenzyme concentrations, pH, EDTA, sodium/potassium chloride, and time, on enzyme activity are presented. Erythrocyte and plasma NADH-GR and NADPH-GR activities in 100 healthy blood donors, and 85 cord blood samples and plasma NADH-GR and NADPH-GR levels in patients with various disease conditions are given.