As part of a programme of comparative measurements of P d (diffusional water permeability) the RBCs (red blood cells) from dingo (Canis familiaris dingo) and greyhound dog (Canis familiaris) were studied. The morphologies of the dingo and greyhound RBCs [examined by light and SEM (scanning electron microscopy)] were found to be very similar, with regard to aspect ratio and size; the mean diameters were estimated to be the same (∼7.2 μm) for both dingo and greyhound RBCs. The water diffusional permeability was monitored by using an Mn2+-doping 1H NMR technique at 400 MHz. The P d (cm/s) values of dingo and greyhound RBCs were similar: 6.5×10−3 at 25°C, 7.5×10−3 at 30°C, 10×10−3 at 37°C and 11.5×10−3 at 42°C. The inhibitory effect of a mercury-containing SH (sulfhydryl)-modifying reagent PCMBS (p-chloromercuribenzene sulfonate) was investigated. The maximal inhibition of dingo and greyhound RBCs was reached in 15–30 min at 37°C with 2 mmol/l PCMBS. The values of maximal inhibition were in the range 72–74% when measured at 25°C and 30°C, and ∼66% at 37°C. The lowest value of P d (corresponding to the basal permeability to water) was ∼2–3×10−3 cm/s in the temperature range 25–37°C. The E a,d (activation energy of water diffusion) was 25 kJ/mol for dingo RBC and 23 kJ/mol for greyhound RBCs. After incubation with PCMBS, the values of E a,d increased, reaching 46–48 kJ/mol in the condition of maximal inhibition of water exchange. The electrophoretograms of membrane polypeptides of the dingo and greyhound RBCs were compared and seen to be very similar. We postulate that the RBC parameters reported in the present study are characteristic of all canine species and, in particular in the two cases presented here, these parameters have not been changed by the peculiar Australian habitat over the millennia (as in the case of the dingo) or over shorter time periods, decades or centuries (as in the case of the domestic greyhound).