1. The transport of the non-metabolized sugar, 3-O-methylglucose, has been studied in the squid axon under conditions where the intracellular environment of the axon is controlled by internal dialysis. 2. Sugar transport is passive, shows saturation kinetics and is asymmetric. At 15 degrees C, the Michaelis and velocity constants for exit are approximately four times those for uptake. The asymmetry of transport is increased by raising the temperature. 3. Sugar uptake is not affected by intracellular sugar levels as high as 100 mM. Sugar exit is, however, reduced by external sugars although the apparent Km for exit is unaffected. 4. The kinetics of sugar exit under exchange conditions are determined by the kinetics of sugar uptake. These results can be accounted for by the asymmetric mobile-carrier and simultaneous-carrier models for transport. 5. Both sugar uptake and exit are reduced in the absence of ATPi. Kinetic analysis of transport under these conditions show that the capacity of the system to transport sugar is unchanged but that the affinity of the system for sugar is reduced. Internal cyclic AMP, AMP, ADP or GTP (2 mM) do not mimic this action of ATP. The hydrolysable analogue of ATP, alpha, beta-methylene-5-ATP (2 mM), (but not the nonhydrolysable analogue beta, gamma-methylene-5-ATP, 2 mM) has an ATP-like action on sugar transport. 6. Transport is unaffected by internal Ca2+ concentrations in the range 4 X 10(-8)--9 X 10(-7) M.