The maintainance of a favorable plant water budget is a condition generally regarded to assure the sequence of cell metabolism. Plant water deficits should be quantitatively evaluated through the determination of tissue water content or tissue water content or tissue water potential. In this work the osmotic potential of stems and leaves, the leaf water potential, and the relative water content-water potential relationships were determined in adequately irrigated patatoes (Solanum tuberosum L.) growing in the field. The results showed that potentials changed during the growing season. Leaf water potential decreseased with aging of the plants and after the complete vegetative growth it was not possible to observe any definite trend, either increasing or decreasing. This change affected relative water content-water potential relationships, so that the relationships shifted to progressively lower water potential for a given relative water content. The data indicated that leaf water potential does not seem a reliable indicator of physiological water deficits. The knowledge of relative water content associated with leaf water potential measurement would provide a more precise condition of water in the plant tissues.