Hemodynamic and myocardial histologic studies have been made in four baboons, each of whom had undergone heart excision and storage, followed by autotransplantation. The excised baboon heart was stored by continuous hypothermic perfusion for 24 or 48 hours and then replaced orthotopically, the baboon being maintained alive in the interim by an orthotopic cardiac allograft. Follow-up of the autotransplanted baboons had been from 3 to 27 months. Cardiac catheterization revealed normal function both early and late after the period of storage. In three animals, myocardial histology was normal on light microscopy, although some dilation of the T tubules were seen on electron microscopy. In the fourth baboon, whose heart has been stored for 48 hours with follow-up for 13 months, light microscopy revealed some variation of staining of the myofibers and mild interstitial edema; ultrastructural studies showed mild intracellular edema, focal scanty loss of myofilaments, and a lack of dilation of the T tubular system. With this one possible exception, this study confirms that the system of hypothermic perfusion storage developed in our laboratory does not have any significant damaging effects on myocardial function or structure. There would appear, therefore, to be no contraindication to its use in clinical heart transplantation.