Effects of potassium supplementation were evaluated in four studies in endurance horses during races and treadmill exercise. In the first and second studies a potassium-free experimental formula was compared to potassium rich commercial formulas. The first study showed that supplementation increased plasma [K+], and that the extra sodium in the potassium-free experimental formulas helped to attenuate acidosis at the end of the ride. In the second study supplementation also increased plasma [K+], however speeds were lower and no increases were observed in plasma concentrations during the race. Supplementation of potassium during recovery helped to restore plasma [K+]. Higher plasma [Ca++] was found in horses supplied with experimental feeds, due to a lower dietary cation anion balance (DCAB). Three eliminated horses had heart rate arrhythmias and labile heart rates accompanied with higher plasma [K+] and lower [Ca++] than finishers. Also horses supplied with the experimental sodium-rich formula were less dehydrated than the ones receiving commercial formulas. The third study involved an 80 km endurance exercise test on the treadmill, and plasma [K+] was affected by potassium supplementation during exercise and recovery. The supply of potassium caused higher plasma [K+] helping to restore body stores. Also chloride supply in the electrolyte formulas maintained plasma [Cl-] levels during exercise and affected plasma concentrations during recovery. The fourth study showed that potassium supply affects plasma concentration, but also increases lactate production and glucose during sub-maximal exercise. A potassium-free electrolyte supply caused higher plasma [Ca++] during exercise. Higher sodium supply in the potassium-free electrolytes improved hydration during exercise. These studies show that potassium should supplemented after exercise and but not be done during exercise because of the risk of increased neuromuscular excitability. / Ph. D.