Summary The rate of water exchange across the erythrocyte membrane was measured using dextran-magnetite as a paramagnetic relaxation agent. The mean residence time of water within the cell was found to be 21.4 ± 0.5(SE) at 22°C. This agreed extremely well with the value measured using manganese. The iron complex was found to not move across the erythrocyte membrane for at least 8 hours, making possible experiments which could not be done using manganese. The activation energy of water transport was measured and this value agreed within 1% of the value using manganese. This suggests that neither reagent induces a substantial perturbation on experimentally determined values of water transport rates.