The reduced form of the tripeptide glutathione has been reported to reinitiate feeding by third-stage larvae of the hookworm Ancylostoma caninum. Investigations were conducted with the GSH analogue ophthalmic acid to determine the mechanism by which GSH stimulates feeding. Ophthalmic acid stimulated feeding, beginning at 1-2.5 mM and reaching a maximum by 10-15 mM, and also enhanced serum-stimulated feeding. The results indicate that GSH stimulates feeding by a specific, sulfhydryl-independent mechanism.