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The purpose of this study was to summarize available information on nutrient excretion by horses from as many sources as possible. Values for amounts and characteristics of fresh manure (feces and urine, as excreted) were obtained from published nutrition study values for nitrogen (N), phosphorus (P) and potassium (K). Only data from horses 18 months of age or older were used. Data were separated into values obtained from non-working (sedentary) and working (exercised) horses. There were insufficient data on growing horses, pregnant mares and lactating mares to evaluate nutrient excretion in these groups. Estimates of dry matter excretion and total manure were calculated using information from the National Research Council "Nutrient Requirements of Horses" and other sources. Equations for estimating N, P, and K excretion were developed from studies reporting nutrient intake and nutrient excretion on a body weight basis. Representative values were calculated using the equations for estimating nutrient excretion for horses weighing 400 to 600 kg.