The objective of this study was to determine the effect of zinc status on plasma glutathione concentration. Immature male rats were fed a low-zinc (less than 1 ppm) diet based on soybean protein. Pair-fed and ad libitum-fed controls consumed a similar diet supplemented with zinc (100 ppm). The plasma concentration of total glutathione (GSH plus GSSG), as determined by the glutathione reductase recycling method of Tietze, decreased in all groups when plasma was allowed to stand in air at room temperature. At 5 min approximately 90% of the total glutathione was present as GSH and the measurable concentration of GSH decreased logarithmically during 60 min. Less than one-half of the loss was accounted for by conversion to GSSG; the nature of the remaining loss is unknown. The plasma of rats fed the zinc-deficient diet for 2 weeks contained less total glutathione than that of ad libitum-fed controls. There was no difference between zinc-deficient rats and pair-fed controls.