The inhibition constant of L-sorbose efflux (Ki(sorbose)) from human erythrocytes for inhibition by D-glucose increases from 5.15 +/- 0.89 to 12.24 +/- 1.9 mM on cooling from 50 degrees C to 30 degrees C; the Ki(sorbose) of D-mannose increases similarly on cooling. The activation energy Ea(sorbose) of net L-sorbose exit from human erythrocytes is 62.9 +/- 3.1 kJ mol-1; but in the co-presence of 5 mM D-glucose Ea(sorbose) is reduced to 41.7 +/- 1.6 kJ mol-1 (P < 0.005). These data are consistent with the view that when D-glucose binds to the hexose transporter it leads to an activated transporter state which remains transiently activated after glucose dissociates; if L-sorbose binds to the excited state it is more mobile than otherwise and consequently the apparent Ki(sorbose) of D-glucose is raised. Cooling prolongs the decay time of the activated state; hence the Ki(sorbose) of D-glucose rises as temperature is reduced.