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1. The resistance of Sub-tropical horses, and desert-dwelling horses to 72 hr dehydration/24 hr rehydration was investigated via changes in red cell parameters and plasma protein concentration 2. Red cell count, haemoglobin and haematocrit increased up to 48 hr dehydration. Between 48 and 72 hr dehydration these parameters decreased, implying a fluid shift onto the intravascular space from the interstitium/hindgut. Most parameters had regained baseline values by 24 hr rehydration.

3. Mean cell volume, mean cell haemoglobin, mean cell haemoglobin concentration and total plasma protein were not significantly different between breeds at, or between most stages of hydration.

4. Protection of plasma volume during dehydration/rehydration was aided by maintaining intravascular protein (especially albumin) levels. Red cells were transiently dehydrated and overhydrated but resisted osmolysis.