The decrease of tissue glutathione (GSH) concentrations in different senescent organisms gave rise to our hypothesis that a glutathione deficiency is a biochemical cause of the aging process. A rigorous test of this notion would be the correction of the deficiency and concomitant increase in life span. To this end, adult mosquitoes were fed magnesium thiazolidine-4-carboxylic acid, and their GSH levels and life spans were determined. The GSH levels increased 50-100% (P less than 0.005) regardless of the age when feeding was initiated or whether the feeding period extended over 2 days or the entire life span. Also the median life spans increased 30-38% over control values (P less than 0.005). The responses were specific for the thiazolidine carboxylate moiety, because MgCl2 had no effect. These findings confirm the GSH deficiency hypothesis and demonstrate a specific biochemical mechanism of aging that can be nutritionally modified.