Preharvest sprouting of wheat (Triticum aestivum L.) involves several plant hormones, but a role for indoleacetic acid (IAA) and its precursor, tryptophan, has not been demonstrated. Our objectives were to determine the roles of IAA, tryptophan, and related compounds in germination of cultivars that differed in susceptibility to preharvest sprouting. L-Tryptophan strongly inhibited germination of embryos excised from caryopses that were highly dormant at harvest but not of embryos from caryopses that had little innate dormancy. The embryos responded similarly to indoleacetaldehyde, IAA, and synthetic auxins, suggesting that tryptophan functioned as a precursor of IAA. Indoleacetaldehyde oxidase inhibitors alleviated the adverse effects of tryptophan and indoleacetaldehyde, and an auxin antagonist decreased the inhibitory action of tryptophan and IAA on embryos from dormant caryopses, further suggesting that IAA was involved. Changes in sensitivity to IAA during afterripening also supported a role for auxin in dormancy. Embryos from caryopses that were highly dormant at harvest gradually lost sensitivity to IAA during afterripening, whereas intact caryopses were insensitive to IAA. The results implicated IAA in dormancy of wheat caryopses and indicated that the auxin might complement the role of abscisic acid in germination. The importance of using dormant caryopses and arresting afterripening in investigations of seed dormancy was noted.