1. Experiments are reported on the entry, exit and exchange fluxes of L-leucine in human erythrocytes in the temperature range 0-37° C. The kinetics of entry and exit become quite different at low temperatures, indicating an asymmetry in the transport mechanism.2. The data is analysed in terms of a simple carrier mechanism. This shows that carrier re-orientation is rate determining at all temperatures studied, and suggests, although does not prove, that the origin of the asymmetry of the carrier may be an unequal equilibrium distribution of the carriers between inward and outward orientations.3. The activation energy increases sharply on going to low temperatures. This increase is the sum of two components. One arises from differences between the activation energies for inward and outward carrier re-orientations, and the other from changes, with temperature, in the activation energies of the individual carrier re-orientation steps.
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