The dose response curve for hydrogen-ion-induced extension growth in Avena coleoptile segments has been reinvestigated. The previously published optimum (pH 3.0) is in error by about two orders of magnitude. The correct optimum is around pH 5.0. This discrepancy is thought to be due to the impermeable nature of the cuticle to hydrogen ions. In the present study the cuticular barrier to H(+) entry was circumvented by using coleoptile segments from which the epidermis with cuticle were physically removed. Using such peeled coleoptile sections, it was also found that auxin can rapidly (20-30 min) initiate H(+) secretion and that the magnitude of auxin-induced secretion is sufficient to initiate considerable cell-extension growth. Furthermore, it is shown that the secretion response is specific for active auxins, and inhibited by agents which inhibit auxin-induced growth (dinitrophenol, abscisic acid, cycloheximide, valinomycin and others). These results make it very likely that H(+) secretion is responsible, at least in part, for the initiation of auxin-induced cell wall loosening and extension growth.