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Effects of lactated Ringer’s solution (LRS) and hetastarch 130/0.4 (HES) on colloid osmotic pressure (COP), plasma osmolality (OSM) and total protein (TP) were investigated in 18 inhalational-anaesthetised healthy horses. Horses received 4–6 ml/kg LRS (LRS; n=9) or HES (HES; n=9) from anaesthesia induction through 60 min, after which all were administered LRS. COP, TP and OSM were measured before premedication (baseline), postinduction and 30 (n=18), 60 (n=18), 90 (n=18) and 120 (n=12) minutes. Baseline COP, OSM and TP were not different between groups. TP decreased in both groups at all time points after induction. OSM increased from baseline in HES at 30, 60, 90 and 120 minutes. COP decreased at 30–120 minutes in LRS, and at 90 and 120 minutes in HES. Mean COP was higher in HES than LRS at 30 (18.8±0.5 vs 16.3±0.4 mmHg (P=0.001)), 60 (19.1±0.5 vs 15.9±0.4 mmHg (P<0.0001)) and 90 (17.4±0.5 vs 15.4±0.5 mmHg (P=0.005)) minutes. Sixty minutes of HES infusion increases OSM and transiently maintains COP compared with an equal volume of LRS in anaesthetised horses.