Conditions necessary to detect maximal auxin-induced H+ secretion using a macroelectrode have been investigated using corn coleoptile segments. Auxin-induced H+ secretion is strongly dependent upon oxygenation or aeration when the tissue to volume ratio is high. Cuticle disruption or removal is also necessary to detect substantial auxin-induced H' secretion. The auxin-induced decrease in pH of the external medium is stronger when the hormone is applied to tissue in which the cuticle has been disrupted with an abrasive than when the hormone is applied to tissue from which the cuticle and epidermis have been removed by peeling. The lower detectable acidification ofthe external medium when using peeled segments appears to be due in part to the leakage of buffers into the medium and in part to the removal of the auxin-sensitive epidermal cells.The sensitivity of corn coleoptile segments to auxin, as measured by H+ secretion, increases about 2-fold during the first 2 hours after excision. Tlhis change in apparent sensitivity to auxin as reflected by H' secretion is paralleled by a time-dependent change in the growth response to auxin. Under optimal conditions for detecting H+ efflux (oxygenation, abrasion, hormone application 2 hours after excision), the latent period in auxininduced H+ efflux (about 7 or 8 minutes) is only half as great as the latent period in auxin-induced growth (about 18 to 20 minutes). These observations are consistent with the acid growth hypothesis of auxin action.According to the acid growth hypothesis of auxin action, auxin enhances growth by stimulating the secretion of H+ ions from the cytoplasm into the cell wall. The resultant acidification of the cell wall is thought to lead to wall loosening and accelerated growth (5,8,13,14).If auxin-induced H+ secretion acts as a mediator of auxininduced growth, one would expect hormone-induced H+ efflux to precede hormone-induced acceleration of growth. The most commonly used method of measuring H+ efflux is to place a large number (e.g. 40) of 1-cm long tissue segments in a small volume (e.g. 5 ml) of stirred weak buffer and monitor the change in pH of the medium in the presence and absence of hormone (12,14). Since the cuticle is said to offer substantial resistance to the entry and exit of H+ ions (3), it is common practice to remove the cuticle either by peeling the tissue segments with a pair of fine-tipped forceps or abrading them with emery powder. Using this method, auxin-induced H+ efflux appears to begin about 20 min or more after application of hormone (1,12,14) long apparent latent period in auxin-induced H+ secretion seems to be due, at least in part, to the large volume of the incubation solution compared with the volume of the cell wall fluid in which H+ would normally be accumulating. When the volume of the external medium is minimized, shorter latent periods are observed. Cleland (2), using a flat-tipped electrode resting on a row of peeled Avena coleoptile sections, reported an average latent period of 14 min in induction ...