A modified perfusion apparatus for electrophysiological, hemodynamic and metabolic studies on hearts of small animal (e.g., rat, guinea pig) has been described. The apparatus consisted of two oxygenators connected to the aortic cannula of the Langendorff perfused heart, each one of which could be used on demand. The constant aortic perfusion pressure was controlled by oxygenating gas (5% CO2 in O2, usually). The perfusing medium could be changed easily and quickly without interruption. Left ventricular pressure was recorded by means of a balloon-catheter, while special suction electrodes obtained the high-amplitude, noise-free electrogram recordings. The coronary effluent partial pressure of oxygen (Po2) was continuously monitored, which enabled the calculation of myocardial oxygen consumption (MVO2). The effluent partial pressure of carbon dioxide (Pco2) and pH value were also measured simultaneously. A computerized system of data acquisition, calculation, storage, and end report has also been described. In this study, the influence of indomethacin on hypoxia-induced hemodynamic and metabolic changes in the isolated guinea pig heart was observed. The advantages and disadvantages of the described and standard Langendorff apparatus have been discussed also.