The characteristics of water diffusional permeability (P) of human red blood cells were studied on isolated erythrocytes by a doping nuclear magnetic resonance technique. In order to estimate the basal permeability the maximal inhibition of water diffusion was induced by exposure of red blood cells to p-chloromercuribenzene sulfonate (PCMBS) under various conditions (concentration, duration, temperature). The lowest values of P were around 0.7×10⁻³ cm s⁻¹ at 10°C, 1.2×10⁻³ cm s⁻¹ at 15°C, 1.4×10⁻³ cm s⁻¹ at 20°C, 1.8×10⁻³ cm s⁻¹ at 25°C, 2.1×10⁻³ cm s⁻¹ at 30°C and 3.5×10⁻³ cm s⁻¹ at 37°C. The mean value of the activation energy of water diffusion (E_a,d) was 25 kJ/mol for control and 43.7 kJ/mol for PCMBS-inhibited erythrocytes. The values of P and E_a,d obtained after induction of maximal inhibition of water diffusion by PCMBS can be taken as references for the basal permeability to water of the human red blood cell membrane.