This study evaluated the effects of pre-exercise frusemide administration and hypertonic saline (H) infusion during anaesthesia on arterial pH, blood gases and electrolytes in 6 Thoroughbred horses anaesthetised immediately post exercise. Horses received either frusemide (F) (0.5 mg/kg, i.v.) or a saline placebo (P) injection 4 h prior to treadmill exercise. Exercise consisted of a 3 min warm-up at 40% V̇O2max followed by exercise to fatigue. Detomidine (40 μg/kg, i.v.) was given within 5 min of stopping and followed 5 min later by zolazepam-tiletamine (1.25 mg/kg, i.v.) to induce anaesthesia. During maintenance of anaesthesia using isoflurane in O2. either isotonic saline solution (I) or 7.5% H (4 ml/kg, i.v.) was administered. Arterial blood was sampled at the following times: prior to (Pre-F/P) and 4 h after (Pre-ex) F or P administration, 1 min after fatigue (Fatigue), 5 min after induction of anaesthesia (Post-ind), and just prior to and 15, 30 and 60 min after beginning isoflurane (ISFL) anaesthesia. Arterial pH and blood gas tensions, pulmonary arterial blood temperature, plasma [lactate], total protein and plasma [Na+], [K+], and [Cl−] were measured. Data were analysed using 2-way ANOVA for repeated measures (P<.05). There were significant differences over time for all variables except total protein. Plasma [Na+] increased at Fatigue compared to Pre-F/P or Pre-ex in all treatment groups. Plasma [Na+] and [Cl−] increased during ISFL anaesthesia compared to Pre-F/P and Pre-ex in horses given H. During ISFL anaesthesia, plasma [Cl−] decreased compared to Pre-F/P in Group F+I horses. We conclude that acid-base, blood gas, and electrolyte changes associated with F are unlikely to influence management of horses anaesthetised immediately following supramaximal exercise to fatigue. Hypertonic saline infusion may help restore plasma [Cl−] faster in hypochloremic horses requiring anaesthesia immediately after strenuous exercise.