Spectrophotometric measurements have been made of the effect of variation of the ionic strength, and equivalent molar concentration of sucrose, on the rate of uptake of CO and O2 by reduced erythrocytes. Cyclic variations of the rate of uptake by as much as 30% have been observed as the ionic strength is altered. The rate of uptake under these conditions is not proportional to the gas concentration. An analysis of these effects suggests the predominant factor is shape-volume changes which alter the membrane permeability. Similar observations have been made on the effect of pH variations. An optimum rate of uptake occurs at pH 7.4. Submitted on September 21, 1961