The effect of a temporary increase in plasma aldosterone concentration on Na⁺, K⁺, and water balance was investigated in four horses. Aldosterone was injected intravenously for 6 h at 20-min intervals (total 5.4 µg/kg body wt). Samples were taken for 24 h before, during, and for 48 h after the treatment. Aldosterone treatment reduced the Na⁺ loss via urine and feces by 99 and 72%, respectively, later followed by a marked increase in Na⁺ excretion by both pathways. During the first 6 h after the treatment, fecal K⁺ excretion was elevated, and the plasma K⁺ concentration was lowered. Fluid was retained throughout the treatment period and for 12-15 h thereafter. In a second experiment, exercise was performed once after aldosterone treatment and once without prior treatment. Sweat samples were collected, and the composition was not altered after treatment. It was concluded that acute aldosterone injections reduce Na⁺ losses in both feces and urine but not in sweat. In addition, the feces was shown to be the main excretion pathway of aldosterone.