The severity of ventricular arrhythmias (number of VPBs, VT episodes, incidences of VT, VF and survival) during the 5 min preconditioning procedure in dogs given either saline (PC) or 5-HD (5-HD +PC). It was difficult to precondition dogs in the presence of 5-HD. In these dogs the number of VPBs and VT episodes, and the incidence of VT, were markedly increased during the 5 min preconditioning occlusion and there was a high incidence of VF both during occlusion and reperfusion. Values are means±s.e.mean. *P<0.05 compared to preconditioned dogs without 5-HD.