The rate of the rapid exchange of formate mediated by band 3 in human erythrocytes, under equilibrium exchange conditions, was measured by using a T1 relaxation method with 13C-labelled formate and 13C NMR, and a pulsed field-gradient spin-echo (PFGSE) method using 1H NMR. The former analysis was based on large differences in T1 between the inside and the outside of the cells brought about by added Mn2+; the latter was based on large differences in the apparent diffusion coefficient inside and outside the cells. There was close agreement in the estimates of the membrane permeabilities made using both methods, suggesting a lack of interference of the exchange process by Mn2+. Regression analysis yielded estimates (under the specified conditions, including 37 degrees C) of Vmax of 3.5 +/- 0.3 x 10(-9) and 3.8 +/- 0.4 x 10(-9) mol cm-2 s-1, and K(m) of 9.8 +/- 0.2 and 8.1 +/- 0.2 mM, for the T1 and the PFGSE methods, respectively. These are new estimates made using methodology that has not previously been applied to measuring rapid (sub-second time scale) formate exchange in cells.