Coronary reserve and contractile reserve were compared between crystalloid-perfused and blood-perfused rabbit hearts at various perfusion pressures (40-110 mmHg). Contractile function of the crystalloid-perfused hearts was dependent on the perfusion pressure, according to Gregg's phenomenon. Developed left ventricular pressure (LVP) increased from 67 +/- 6 mmHg to 121 +/- 5 mmHg and positive dP/dt maximum (dP/dt max) from 1,083 +/- 75 to 2,233 +/- 126 mmHg/s at perfusion pressures between the lowest and highest perfusion pressure. In the blood-perfused hearts, the perfusion pressure-induced changes were less pronounced: developed LVP changed from 107 +/- 11 mmHg to 138 +/- 8 mmHg and dP/dt max from 1,517 +/- 181 to 2,008 +/- 187 mmHg/s. The blood-perfused hearts showed better cardiac function, especially negative dP/dt minimum (dP/dt min), compared to the crystalloid-perfused hearts. Contractile reserve estimated by paired pacing technique was quite independent of the perfusion pressure in the blood-perfused hearts but not in the crystalloid-perfused hearts, and was significantly better in the blood-perfused hearts (e.g., 81% increase of developed LVP with blood perfusion, and 26% increase with crystalloid perfusion at a perfusion pressure of 80 mmHg). Coronary reserve, estimated by reactive hyperemia, was independent of the perfusion pressure in both groups. Coronary reserve was small in the crystalloid-perfused hearts (< 23%) and more than double the control value in the blood-perfused hearts. It is proposed that blood-perfused hearts are more suitable for physiological and pathophysiological studies.