Selected constituents of the blood,urine and faecal fluids of Shetland and Shetland -cross ponies were studied and considerable variations between and within individual ponies were observed. Wide ranges of values were observed in the untreated ponies, and thus it is likely that erroneous judgements could result from attempts to assess normality or abnormality on the basis of a single observation. The packed cell volume percentages of these ponies were similar to those reported in other Shetland -type ponies and draught horses. Factors affecting the packed cell volume percentage both in vivo and in vitro are discussed. The plasma sodium concentration values of the ponies were lower than those found in other equines. The daily water loss in urine was approximately equal to the water lost in faeces. Of the urinary and faecal fluid constituents studied only inorganic phosphate was lost in greater quantities in the faecal fluid than in the urine. The estimated daily intake of sodium, potassium and chloride was compared with daily losses in urine and faecal fluid, and in all cases daily losses in urine and faecal fluids were less than the quantities ingested. The concentrations of selected blood constituents were monitored hourly for eight hours before the ponies were presented with their daily hay ration, and for eight hours after they commenced feeding. After feeding began increases in the packed cell volume percentage and in the concentrations in plasma of urea and sodium were observed. Some decreases in plasma chloride and inorganic phosphate concentrations occurred. Thus it was evident that standardisation of the time of blood sampling in relation to feeding time was essential when the effect of an experimental treatment upon these blood constituents was to be studied. The plasma and "thiocyanate space" volumes were measured by the dilution of T-1824 and sodium thiocyanate respectively, and the approximate total quantities of certain plasma constituents were estimated. The oral administration of 7.5 litres (35 46 ml /kg body weight) of water caused increases in the plasma volume of 1.4 to 8.6 %, and in the "thiocyanate space" volume of 7.0 > to 21.6 %. Fluctuations in the packed cell volume percentage and the concentration of selected plasma constituents did not reflect the magnitude and time of plasma volume expansion indicated by T- 1824. Changes in the volume and composition of the urine voided after water loading are described and discussed. The ingestion of ammonium chloride induced a meta - bolic acidosis with concurrent respiratory compensation. The clinical signs of metabolic acidosis were evident. Urinary pH decreased and a change in urinary excretion of net base to net acid took place. The ponies were able to excrete net acid even when the pH of the urine was greater than that of the blood. Ammonium chloride ingestion induced natriuresis without diuresis or hyponatraemia. Other changes in the blood and urine constituents outwith the normal ranges are described and discussed. The administration of sodium bicarbonate in a dose equivalent to that of ammonium chloride induced few changes in blood acid /base parameters. Although urinary net base excretion increased markedly no significant increases in urine pH were detected, and hence net acid /base excretion was believed to be a more accurate indication of the acid base status of the pony and the renal response to acid/ base disturbances than the pH of the urine. Since both ammonium chloride and sodium bicarbonate induced natriuresis without diuresis it was apparent that this phenomenon could arise regardless of whether the source of sodium was exogenous or endogenous. A possible relationship between urinary sodium and inorganic phosphate excretion is discussed. Changes outwith the norm of other blood and urine constituents are also described and discussed.