Previous studies have shown that preconditioning (PC) with a brief ischemic episode induces heat shock protein (HSP) in cardiac tissue. However, it is unclear when and where in the left ventricle HSP is expressed after PC. Hence, the expression of HSP was studied in rabbit hearts at various time intervals after PC using immunohistochemical methods. Rabbits were preconditioned four times with 5 min of occlusion and 5 min of reperfusion of the coronary artery and then were killed at 0, 3, 24, 48, 72 and 168 h after the PC (n=4, for each time interval). Samples were obtained from the subendocardium and subepicardium of the preconditioned and nonpreconditioned wall and these were processed to 4 microm thick cryosections. The sections were immunolabelled with mouse monoclonal IgGs against HSP 72/73. Positive immunoreactivity was observed as early as 3 h after PC, persisting up to 72 h but not detected at 168 h. HSP was expressed not only in the preconditioned myocardium but also in the remote nonpreconditioned myocardium. There was a wide variation in expression among myocytes. Expression was dominant in myocytes compared with vessel walls. It was concluded that PC induced transient and inhomogeneous expression of HSP in rabbit hearts.