As part of a programme of comparative measurements of P d (diffusional water permeability) the RBCs (red blood cells) from an aquatic monotreme, platypus (Ornithorhynchus anatinus), and an aquatic reptile, saltwater crocodile (Crocodylus porosus) were studied. The mean diameter of platypus RBCs was estimated by light microscopy and found to be ∼6.3 μm. P d was measured by using an Mn2+-doping 1H NMR (nuclear magnetic resonance) technique. The P d (cm/s) values were relatively low: ∼2.1×10−3 at 25°C, 2.5×10−3 at 30°C, 3.4×10−3 at 37°C and 4.5 at 42°C for the platypus RBCs and ∼2.8×10−3 at 25°C, 3.2×10−3 at 30°C, 4.5×10−3 at 37°C and 5.7×10−3 at 42°C for the crocodile RBCs. In parallel with the low water permeability, the E a,d (activation energy of water diffusion) was relatively high, ∼35 kJ/mol. These results suggest that “conventional” WCPs (water channel proteins), or AQPs (aquaporins), are probably absent from the plasma membranes of RBCs from both the platypus and the saltwater crocodile.