Incubation of Zea coleoptiles in 0.5 M mannitol totally inhibits growth and geotropic curvature, but does not affect the development of the geoelectric effect. This pre-treatment also inhibits the curvature induced by the asymmetrical application of IAA to the apical end of decapitated vertical coleoptiles, but it does not prevent the IAA from giving rise to an electropotential difference between the two sides of the coleoptile. Neither the normal geoelectric effect, nor the auxin-induced potential difference in vertical coleoptiles, can therefore arise as the result of the different rates of cell extension in the two halves of the organ. They must be the result of the change of IAA concentration affecting some other aspect of the cell's physiology or metabolism. The abolition of the electrical responses in coleoptiles which have been plasmolysed in 1.0 M mannitol strongly suggests that both longitudinal and lateral transport of IAA are severely depressed by this degree of plasmolysis. Asymmetrical application of 10-5 M mersalyl and several other substances to the apical end of a decapitated vertical coleoptile gave rise to a marked electropotential difference between the two sides of the coleoptile, the side beneath the donor being positively charged with respect to the other side. Mersalyl does not promote the growth of Zea coleoptiles. These results provide additional evidence that the electropotentials do not arise from differential growth, and suggest that such substances, especially the diuretics used in clinical medicine, may provide useful tools in the further study of the induction of surface electropotentials in plant tissues at the cellular level.