Prevalence of hyponatraemia has not been extensively studied in pet rabbits, and the reference data for calculated plasma tonicity and osmolarity are not available. This retrospective clinical study reports the prevalence of hyponatraemia, hyposmolarity and hypotonicity in ill pet rabbits (n=356). The relationship between sodium and glucose levels was studied (n=134). Mortality rates within seven days associated with different sodium levels were calculated in ill rabbits (n=322). Venous blood samples in lithium heparin were processed using iStat EC8+ cartridges. The 95% RI for plasma sodium, calculated osmolarity and tonicity from 51 healthy pet rabbits were 136–147 mEq/l, 284–312 mOsm/l and 278–302 mOsm/l, respectively. The prevalence of hyponatraemia, hypotonicity and hyposmolarity was 39.0 per cent, 28.7 per cent and 18.0 per cent, respectively. Pseudohyponatraemia was present in 28.1 per cent and true hyponatraemia was present in 71.9 per cent of the cases of hyponatraemia. Sodium levels less than 129 mEq/l were found to be associated with 2.3-fold increase in mortality risk. Plasmatic sodium levels in rabbits decrease in conditions of hyperglycaemia in a similar manner as it occurs in human beings. As hyperglycaemia is quite a common condition in rabbits, simultaneous measurement of plasmatic sodium along with glucose in ill rabbits is advised. Hyponatraemia is a common condition in ill rabbits and, depending on its type (true hyponatraemia or pseudohyponatraemia), of varying clinical relevance. Calculation of plasmatic tonicity is necessary for differentiation of types of hyponatraemia.