Oxygen uptake into intact and reconstituted human red blood cells was measured using dual wavelength, stopped flow techniques. The rate of oxygen uptake by human erythrocytes is roughly 40 times slower (t 1/2 congruent to 80 ms at 0.125 mM O2, 25 degrees C) than the corresponding rate of oxygen combination with free hemoglobin. Oxygen transport through the red cell cytoplasm accounts for part of this difference and predicts a half-time of uptake of about 15 ms, which is still 5 times smaller than that observed experimentally. Further limitation of uptake appears to be due to the presence of unstirred layers of solvent adjacent to the red cell surface. Very rapidly after mixing, these layers form and become depleted of O2 due to uptake by the cells. This requires that the bulk of the oxygen molecules must diffuse over rather large distances, 1.0 to 5.0 micrometer, before they can penetrate the erythrocytes. A mathematical model was developed to take into account diffusion through an unstirred solvent layer which increases in thickness with time. This scheme can account quantitatively both for the dependence of the apparent rate of uptake on O2 concentration and for the shape of the observed time courses. In addition, the diffusion parameters which were developed for the O2 reaction can also be used to describe quantitatively the rates and time courses of CO and ethyl isocyanide uptake and the rates and time courses of O2 release from cells in the presence of sodium dithionite. Finally, the parameters used to describe the stopped flow results can also be used to simulate quantitatively O2 uptake time courses obtained from previous studies with thin films of red cells (Sinha, A. K. (1969) Ph.D. dissertation, University of California, San Francisco; Thews, G. (1959) Arch. Gesamte Physiol. Mens. Tiere (Pflufgers) 268, 308-317).