1. The water permeabilities of the red blood cell (RBC) membranes of five species of macropodid marsupials were monitored by using a Mn(2+)-doping 1H nuclear magnetic resonance (NMR) technique. 2. Since this appears to be the first time that this type of measurement at 400 MHz for 1H has been reported, an analysis of instrumental parameters influencing the estimated value of the water exchange time (Te), and of the diffusional water permeability (Pd), was performed on samples of human RBC. 3. It was found that a short interpulse delay in the Carr-Purcell-Meiboom-Gill pulse sequence had to be used (around 100 microseconds) to avoid an underestimation of the relaxation times, that occurred as the result of molecular diffusion through non-uniform local magnetic fields in and around the cells. 4. There were no significant differences, in the water permeabilities of the RBC membranes, between the five species (Macropus rufogriseus, M. parma, M. eugenii, M. parryi and Wallabia bicolor). 5. There were also no significant differences between various colonies of M. eugenii living in different habitats. 6. The average values of Pd were 9.1 x 10(-3) cm/sec at 24.6 degrees C, 10.4 x 10(-3) cm/sec at 30 degrees C, 12.6 x 10(-3) cm/sec at 37 degrees C, and 14.7 x 10(-3) cm/sec at 42.1 degrees C; these were more than twice as high as those for human RBC. 7. In agreement with the high water permeability the RBC of macropodids displayed a relatively low activation energy of water diffusion across their membranes, approximately 21 kJ/mol, compared with 25 kJ/mol for human RBC.