AbstractThe rate of uridine uptake was measured inTetrahymenaafter shiftdown to non‐nutrient physiological salt solution. Uptake follows Michaelis‐Menten kinetics and an apparent Kmof transport of 2 × 10−6M has been estimated. This value is in good agreement with those reported for tissue‐derived cells in culture. Incorporation of uridine into RNA follows similar kinetics suggesting that uptake is rate limiting for incorporation. Within three hours after shiftdown the rate of uptake is decreased by an order of magnitude. Also at three hours after shiftdown pairing occurs between cells of complementary mating types. It seems likely that the change in uptake is a reflection of a surface change associated with differentiation. The rate of uptake was also measured during the interdivision period using cells synchronized by a physical selection procedure. A change in rate occurs at the time the cells begin replication of DNA and is essentially stable thereafter. These results indicate that there exists inTetrahymenaa relationship between surface properties as assayed by uridine uptake and properties of growth and differe
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