We assessed the effect of external mechanical work (EW) during the relaxation period (RP) on myocardial oxygen consumption (VO2) and clarified the energetic significance of the potential energy (PE) portion of the pressure-volume area (PVA) in the cross-circulated dog left ventricle. We changed the course of the relaxation segment of the pressure-volume (P-V) trajectory by increasing or decreasing EW within a given PVA without changing the end-diastolic volume (EDV) and the systolic segment of the P-V trajectory while measuring VO2. Thus the ventricle underwent ejection or filling during RP. Although the percent fraction of EW in PVA (%EW/PVA) was markedly increased from 32 +/- 12 (SD) to 93 +/- 3% in ejecting contractions (8 hearts) and from 0 to 93 +/- 5% in isovolumic contractions (3 hearts), these marked changes in %EW/PVA did not significantly affect VO2. Moreover, the VO2-PVA data during these procedures fell on the reference VO2-PVA relation line obtained by changing EDV and PVA of isovolumic contractions. We conclude that EW during RP at a constant PVA does not affect VO2 and part of PE can be converted into EW in an energetically equivalent manner.