Rayless goldenrod (Isocoma pluriflora) has been known to be toxic to livestock in the southwestern United States for many years; however, chemical composition of the plant as well as the dosage and duration required to cause toxicosis have not been completely described. Tremetol, the historical toxin, is actually a mixture of alcohols and ketones. Though not completely confirmed experimentally, the toxic compounds are believed to be benzofuran ketones that include tremetone, dehydrotremetone, 3-hydroxytremetone, and 3-oxyangeloyl-tremetone. The objectives of this study were to determine the dosage of benzofuran ketones and the duration of exposure to these compounds required to produce clinical signs of poisoning in Spanish goats and to document the pathophysiological changes associated with rayless goldenrod poisoning in goats. Goats dosed with rayless goldenrod containing 40 and 60 mg/kg BW of benzofuran ketones for 4 or 5 days, showed clinical signs of toxicosis that included trembles, and exercise intolerance seen as reluctance to perform on the treadmill, significantly increased resting and working heart rates and prolonged heart rate recovery following exercise. The affected goats also had significant serum biochemical changes that included increased concentrations of cardiac troponin I and increased activities of aspartate aminotransferase, alanine aminotransferase, lactate dehydrogenase, and creatine kinase. Exercise intolerant animals also had extensive degeneration and necrosis within nearly all skeletal muscles. Some goats dosed with 10 and 20 mg/kg BW of benzofuran ketones began to show some signs of poisoning on the last day of the study. In conclusion, benzofuran ketones at doses at or above 40 mg/kg BW for longer than 4 or 5 days are toxic and produce disease similar to that described in clinical rayless goldenrod poisoning. Additionally, smaller benzofuran ketone doses (10 and 20 mg/kg BW) for longer durations also cause the disease. The physiologic findings indicated that though there may be some myocardial changes, the majority of the clinical disease in goats is due to skeletal muscle degeneration and necrosis. More work is needed to determine the toxicity and physiologic effects of individual benzofuran ketones and to develop a model that better predicts the risk of poisoning and methods to avoid poisoning by plants containing benzofuran ketones.