1. A method was developed which allowed the measurement of sugar uptake into red cells after only 4 sec.2. 3. For these conditions, we found linear plots against time, which enabled us to determine the initial velocity for the uptake of label, i.e. the unidirectional influx.4. Influx depended on the widely varying cis-concentration of glucose in the medium in the manner of a simple saturation-relationship. The parameters J,,, and K m , determined from a reciprocal plot, were different for the three chosen [St,]-conditions. Some of these parameters were comparable to values of other papers, determined by measuring the efflux of glucose from red cells. 5.The results were compared with predictions derived from two models of a mobile carrier: the symmetrical model of Levine, Oxender and Stein [I] and the asymmetrical, but passive model of Geck [2]. The relationship between glucose uptake plotted against time and influx plotted against cis-concentration corresponded to both models. Some, but not all, ratios between the measured parameters matched the predictions of the symmetrical model, whereas all ratios were compatible with the asymmetrical model. 6. From both models, the translocation resistance comes out to be much greater for the free carrier than for the glucose-carrier complex: 10 to 30 fold a t 20 "C and circa 200 fold a t 0 "C. Hence, the mobility of the free carrier appears to be very much reduced in the cold.