A method of monitoring was developed to directly measure the intramyocardial pressure and to objectively assess the viability and contractility of a heart allograft before it is harvested, during its period of preservation and following its implantation. Intramyocardial pressure was measured in the subendocardial and subepicardial regions using implantable solid state sensors. The data demonstrated that a normally contracting in situ heart exhibits a transmural intramyocardial pressure gradient, the systolic subendocardial pressure being consistently greater than the left ventricle and subepicardial pressures. Subendocardial pressure markedly changes during inotropic stimulation or myocardial ischemia. In three canine allografts and in an isolated, perfused and vented beating heart similar responses were observed during pharmacologic and hemodynamic testing. The intramyocardial pressure measurement proved to be relatively insensitive to preload and afterload changes provided coronary perfusion remained unaltered. Ventricular fibrillation produced an elevated and oscillating intramyocardial pressure while cardioplegic arrest reduced it to near zero. Diastolic pressure measurements were most sensitive to detect myocardial contracture ("stone" heart) during which intramyocardial pressure increased significantly. The "stone" heart exhibited persistent mechanical activity despite no visible contraction. The edematous heart's response to inotropic stimulation was reduced. Ischemia induced by inadequate perfusion was detected by a rapid drop in systolic intramyocardial pressure, preferentially affecting the endocardial region. This study establishes that the change in diastolic intramyocardial pressures in response in inotropic stimulus is a reliable indicator of myocardial contractility and viability and could be used during the procurement and preservation of the heart for transplantation.