The temperature-dependence of water diffusion across human erythrocyte membrane was studied on isolated erythrocytes and resealed ghosts by a doping nuclear magnetic resonance technique. The conclusions are the following: (1) The storage of suspended erythrocytes at 2 degrees C up to 24 h or at 37 degrees C for 30 min did not change the water exchange time significantly, even if Mn2+ was present in the medium. This indicates that no significant penetration of Mn2+ is taking place under such conditions. (2) In case of cells previously incubated at 37 degrees C for longer than 30 min with concentrations of p-chloromercuribenzene sulfonate (PCMBS) greater than 0.5 mM, the water-exchange time gradually decreased if the cells were stored in the presence of Mn2+ for more than 10 min at 37 degrees C. (3) When the Arrhenius plot of the water-exchange time was calculated on the basis of measurements performed in such a way as to avoid a prolonged exposure of erythrocytes to Mn2+ no discontinuity occurred, regardless of the treatment with PCMBS. (4) No significant differences between erythrocytes and resealed ghosts regarding their permeability and the activation energy of water diffusion (Ea,d) were noticed. The mean value of Ea,d obtained on erythrocytes from 35 donors was 24.5 kJ/mol. (5) The value of Ea,d increased after treatment with PCMBS, in parallel with the percentage inhibition of water diffusion. A mean value of 41.3 kJ/mol was obtained for Ea,d of erythrocytes incubated with 1 mM PCMBS for 60 min at 37 degrees C and 28.3 kJ/mol for ghosts incubated with 0.1 mM PCMBS for 15 min, the values of inhibition being 46% and 21% respectively.