The effects of an oral isoxsuprine-resin preparation on the blood flow in the thoracic limb of seven horses was determined by thermography. Treatment with the oral resin preparation resulted in increased skin temperatures compared with the non-medicated controls. The maximal temperature differences, 2.2 degrees C for the horses treated with 0.9 mg/kg and 1.8 degrees C for the horses treated with 1.2 mg/kg, occurred four hours after dosing. Plasma total isoxsuprine, determined in three horses, was detectable two hours after oral dosing and maximal eight hours after dosing, but free isoxsuprine could not be detected. Receptor binding studies demonstrated strong alpha-receptor binding, and this binding was so strong that even at isoxsuprine concentrations below the detection level receptors could have been stimulated.