Trabeculae isolated from human atrial appendages have been used to study preservation of donor hearts for cardiac transplantation. We have developed a perifusion system equipped with a fiber optic strain gauge to study mechanical performance of human atrial trabeculae (10-20 mg) while simultaneously observing the energetic compounds by 31P NMR spectroscopy. The NMR probe consists of an eight-turn solenoid coil (2.3 mm i.d. x 5 mm length, 24-gauge wire) double tuned to allow observation of 1H and 31P nuclei. The probe and the perifusion system are temperature regulated and permit preservation studies using a variety of small muscles at low temperatures (down to 4 degrees C) as well as at physiological temperature (37 degrees C). 31P NMR spectra suitable for quantification can be obtained from approximately 10 mg of human atrial trabeculae in 15 min. Spectra of 8- to 12-mg mouse extensor digitalis longus or soleus muscle can be obtained in less than 10 min.