Studies with mammals and birds clearly demonstrate that brief preexposure to oxygen deprivation can protect the myocardium from damage normally associated with a subsequent prolonged hypoxic/ischemic episode. However, is not known whether this potent mechanism of myocardial protection, termed preconditioning, exists in other vertebrates including fishes. In this study, we used an in situ trout (Oncorhynchus mykiss) working heart preparation at 10 degrees C to examine whether prior exposure to 5 min of anoxia (PO(2) < or = 5 mmHg) could reduce or eliminate the myocardial dysfunction that normally follows 15 min of anoxic exposure. Hearts were exposed either to a control treatment (oxygenated perfusion) or to one of three anoxic treatments: 1) anoxia with low P(out) [15 min of anoxia at an output pressure (P(out)) of 10 cmH(2)O]; 2) anoxia with high P(out) [10 min of anoxia at a P(out) of 10 cmH(2)O, followed by 5 min of anoxia at P(out) = 50 cmH(2)O]; and 3) preconditioning [5 min of anoxia at P(out) = 10 cmH(2)O, followed after 20 min of oxygenated perfusion by the protocol described for the anoxia with high P(out) group]. Changes in maximum cardiac function, measured before and after anoxic exposure, were used to assess myocardial damage. Maximum cardiac performance of the control group was unaffected by the experimental protocol, whereas 15 min of anoxia at low P(out) decreased maximum stroke volume (V(s max)) by 15% and maximum cardiac output (Q(max)) by 23%. When the anoxic workload was increased by raising P(out) to 50 cmH(2)O, these parameters were decreased further (by 23 and 38%, respectively). Preconditioning with anoxia completely prevented the reductions in V(s max) and Q(max) that were observed in the anoxia with high P(out) group and any anoxia-related increases in the input pressure (P(in)) required to maintain resting Q (16 ml. min(-1). kg(-1)). Myocardial levels of glycogen and lactate were not affected by any of the experimental treatments; however, lactate efflux was sevenfold higher in the preconditioned hearts. These data strongly suggest that 1) a preconditioning-like mechanism exists in the rainbow trout heart, 2) increased anaerobic glycolysis, fueled by exogenous glucose, was associated with anoxic preconditioning, and 3) preconditioning represents a fundamental mechanism of cardioprotection that appeared early in the evolution of vertebrates.