Inhibition by cycloheximide of sulfate transport in both barley roots (Hordeum vulgare L.) and potato tuber (Solanum tuberosum L.) increases with increasing inhibitor concentration only to a limited extent, depending on the length of the tissue incubation with the inhibitor. In contrast to this, increasing concentrations of dinitrophenol have a rapid and total inhibitory effect on the active transport. Leucine transport in the same tissues is strongly inhibited by dinitrophenol but is not affected by cycloheximide, whereas incorporation into protein is mainly inhibited by cycloheximide. It appears that the mechanism of transport inhibition by cycloheximide in plant tissues consists in stopping new carrier synthesis and not in the disruption of energy flow. Sulfate carriers show comparable decay rates in barley roots and potato tuber, the mean life being shorter than that of the leucine carriers. These appear more stable in roots than in storage tissues.