Cavitation is implicated as the cause of pitting and erosion of explanted mechanical heart valves that failed. Previous in vitro studies demonstrated transient negative pressure spikes upstream of mechanical heart valves at the instant of leaflet closure. When the magnitude of the transient negative pressure spike is below the vapor pressure of the fluid flowing across the mechanical valve, cavitation bubbles have been documented near the valve housing or occluder disc. To test for the presence of transient negative pressure spikes that are conducive to cavitation in vivo, we measured left atrial pressure at the valve orifice after mitral valve replacement. Mitral valves were replaced with 27 mm prostheses in 10 goats (50-60 kg). Control animals (Group 1, n = 5) received pericardial valves. Study animals (Group 2, n = 5) received bileaflet pyrolytic carbon valves. Pressure was recorded from a high frequency atrial transducer at hyperdynamic and hypodynamic states. Transient negative pressure spikes did not occur in any Group 1 animal. Transient negative pressure spikes below the vapor pressure of blood (-713 mm Hg) were recorded in four of five Group 2 animals at the hyperdynamic state: -900, -950, -800, -400, and -1,400 mm Hg (p = 0.048 Group 1 versus Group 2, Fisher's exact test). No cavitation potential exists in vivo after bioprosthetic valve implantation. Transient negative pressure spikes below the vapor pressure of blood occur in vivo at hyperdynamic physiologic states when this bileaflet pyrolytic carbon valve is implanted in the mitral position. These studies demonstrate the potential for cavitation with implanted mechanical valves in vivo.