The influence of light and dark on the respiration rate, sugar level, and rubidium uptake of the roots of young sunflower and barley plants has been investigated. The rate of root respiration was found to be closely correlated with the rate of carbohydrate transport from the shoots. This enabled the energy required for root growth to be calculated. A period of darkness caused a reduction in rubidium uptake and transport which appeared to be due to the reduced rate of root metabolism in the dark. The experiments of some earlier workers on the effect of transpiration on rubidium uptake by young sunflower and barley plants were repeated. The results indicated that they were studying the metabolic effect of light and dark on salt uptake as well as the effect of transpiration.