We studied the effects of a single ketamine injection in an experimental model of chronic hypernatraemia in rats. Three groups, each of 20 male Wistar rats were chosen for the experiment; the control group was given water and the test groups were given 1% (group 1) and 2% (group 2) NaCl solutions for two weeks. All groups were fed with the same diet, containing about 0.5% salt. Other living conditions for all groups were similar. Before using saline in the test groups and before induction of anaesthesia, blood samples were drawn to measure the serum sodium level. A single ketamine injection (125 mg/kg, IP) was used in all groups. Latency times to inhibit the righting reflex and to inhibit the response to painful stimulus, re-appearance time of response to painful stimulus and recovery time from anaesthesia were measured; mortality rates during anaesthesia were also recorded. During consumption of salt solutions and before induction of anaesthesia, we had no animal death. The serum sodium level in group 2 was significantly higher than group 1 and the control group. The required time to inhibit the righting reflex and the response to painful stimulus in group 2 was significantly shorter than group 1 and the control group. These parameters in group 1 were also significantly shorter than the control group. The required time for re-appearance of response to painful stimulus and for recovery from anaesthesia in group 2 was significantly longer than group 1 and the control group. We observed severe pulmonary complications in the test groups during anaesthesia. Mortality rate in group 2 was 60% and in group 1 was 20%; the control group had no significant problems during anaesthesia. Hypernatraemia decreased the induction time of anaesthesia. The duration of ketamine anaesthesia increased and the recovery from anaesthesia was associated with significant delay.