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The saturable, 'carrier-mediated' pathway for sugar transport in pigeon red blood cells may be stimulated by metabolic depletion or by loading the cells with Ca by means of a high-voltage discharge or the ionophore A 23187. All of these methods for stimulating the saturable pathway of sugar transport also cause a drop in cellular ATP levels. The relationship between the stimulation of transport and the fall in ATP is very similar in metabolically depleted or Ca-loaded cells, but cells treated with Ca and A23187 show a greater stimulation of transport than would be expected from the decline in ATP. Altering free Ca2+ levels during metabolic depletion has little or no effect on stimulation of the saturable pathway. Conversely, metabolic depletion of fresh cells in Ca-free solutions has no detectable effect on intracellular free Ca2+ levels. These results suggest that Ca2+ ions are not involved in regulation of this pathway. The non-saturable pathway for sugar transport is stimulated by a rise in cell Ca. This process is probably stimulated half-maximally by about 10 microM-free Ca2+.