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Inosine uptake into membrane vesicles prepared from baby hamster kidney (BHK) cells appears to proceed by two distinct mechanisms. One mechanism results in the accumulation of ribose 1-phosphate (ribose-l-P) while hypoxanthine is left in the medium. It has a low Km and a high initial rate. Since inosine is cleaved, it is a purine nucleoside phosphorylase dependent step. The second mechanism is purine nucleoside phosphorylase independent and results in intravesicular accumulation of intact inosine. The resultant inosine is metabolized by a purine nucleoside phosphorylase activity, which may or may not be the same as that used in the first mechanism which resulted in extravesicular hypoxanthine, leading to hypoxanthine and additional ribose-l-P. The initial I^evious studies from this laboratory have shown that, when inosine interacts with isolated plasma membrane vesicles from cultured mouse fibroblast cells grown to high cell density, the predominant intravesicular transport product is ribose-l-P1 (Li and Hochstadt, 1976a,b;Hochstadt and Quinlan, 1976). The mechanism involves membrane-localized purine nucleoside phosphorylase acting in a group translocation reaction during which hypoxanthine is released on the exterior membrane surface while the ribose moiety is phosphorylated in the process of being transported across the membrane.In contrast, in polyoma transformed baby hamster kidney (BHK) cells there exist two mechanisms of inosine handling: one, similar to that we have described for the mouse fibroblast cells, in which group translocation occurs and depends on a transmembranal purine nucleoside phosphorylase and for which internal inosine is not a substrate (Hochstadt and Quinlan, 1976;Li and Hochstadt, 1976b) and a second mechanism which involves the uptake of intact inosine. The