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The kinetics of 3-O-methylglucose transport in the giant muscle cells of Balanus nubilus have been studied both in intact fibres and in fibres subjected to intracellular solute control using internal dialysis. 3-O-methylglucose is not metabolized by barnacle muscle and at equilibrium the 3-O-methylglucose space of the tissue does not differ significantly from the water content of the muscle. These results indicate that 3-O-methylglucose transfer in barnacle muscle is mediated by a passive process. 3-O-methylglucose transport is facilitated by a saturable, symmetric transfer mechanism inhibited by cis but not trans sugars and by low concentrations of phloretin and cytochalasin B. The kinetic constants for uptake and exit are identical. These features indicate that sugar transport in barnacle muscle is mediated by a limited number of membrane transport sites. The number of sugar-displaceable cytochalasin B binding sites in barnacle muscle is 3 X 10(13) cm-2. Indirect kinetic estimates indicate that the number of sugar transport sites is in the order of 1.6 X 10(12) cm-2. This passive, facilitated, selective, saturable transport system is consistent with both symmetric mobile carrier (one-site) and symmetric simultaneous carrier (two-site) models for transport.