Goose red blood cells were studied as a model for metabolic regulation of sugar transport. In contrast to their action in human erythrocytes, sulfhydryl-blocking agents such as N-ethylmaleimide (NEM) stimulated 3-O-methylglucose transport markedly in goose red blood cells. The effect of NEM was further enhanced when adenosine 5'-triphosphate (ATP) was first depleted by 2,4-dinitrophenol treatment or anoxia. Only sulfhydryl-blocking agents that enter the cell were effective transport stimulators, and the effect was not altered by substrates of the transporter. In nucleated red blood cell ghosts, NEM inhibited 3-O-methylglucose transport. Results of these studies with intact cells were consistent with the hypothesis that free sulfhydryl groups are essential for regulation of transporter activity rather than for the transport process itself. The locus of NEM action appears to be either on the cytoplasmic side of the membrane or partially located in the cytoplasm. ATP depletion may expose previously masked sulfhydryl groups, producing an enhanced reaction with sulfhydryl-blocking agents and a highly stimulated rate of sugar transport.