1. Erythrocyte cation transport pathways have been investigated in a family with pseudohyperkalaemia.
2. Ouabain- and bumetanide-resistant Na+ and K+ effluxes in three pseudohyperkalaemic patients were not different from those of control subjects when assessed at 37°C.
3. When the temperature was decreased to 20°C and 9°C, K+ passive permeability markedly increased and Na+ permeability remained unchanged in these patients. In contrast, in control subjects a reduction in temperature caused a marked reduction in Na+ and K+ passive permeability.
4. These findings could account for the marked increase in plasma K+ concentration observed at sub-physiological temperatures.
5. The Na+−K+ co-transport pathway was reduced in all members of the family, but the Na+−K+ pump was reduced in only two of them. These alterations were independent from the pseudohyperkalaemic state.