Although some of the physiological mechanisms which contribute to salt tolerance in plants are known, there are stillsome major gaps in understanding and it remains impossible to provide a satisfactory integrated picture for the plantas a whole. The operation of stomata in halophytes has received little attention even though all of the salt presentin the shoot (apart from that taken in during submergence) is thought to be carried in the transpiration stream. Innon-halophytes, stomatal function is damaged by sodium ions, and disruption of the normal regulation oftranspiration should be seen as a possible contributor to their inability to survive in salt-laden soils. The developmentof salt-tolerant cultivars of crops may require attention to the need for appropriate adaptations to the ionic relationsof stomatal guard cells. Despite the small amount of evidence available, it is possible to identify two alternativeadaptations that occur in the stomata of halophytes : (1) the guard cells can utilize Na+instead of K+to achieve theirnormal regulation of turgor ; (2) the guard cells continue to use K+and are able to limit their intake of Na+. Thesecond adaptation is worthy of further exploration because it may provide a means for ` top down ' control oftranspiration and, therefore, of the amount of salt delivered to the shoot. This mechanism may be very important insome of the glandless halophytes, and it could be of particular interest as a potential contributor to the developmentof salt tolerance in crops.