We examined the in vivo effect of acute hypoxemia on myocardial cell-surface (sarcolemmal) -adrenoreceptor density (B_max) and binding affinity (K_D) and on stress protein 70 (sp70) expression by exposing rainbow trout (Oncorhynchus mykiss; 2.1-2.7 kg) to hypoxic water (3 mg/l O₂) at 15°C for 6 h. This degree of hypoxia was the minimum O₂ level that these trout could tolerate without losing equilibrium and struggling violently. Hypoxic exposure reduced arterial PO₂ (Pa_O2) from 98 to 26 mmHg and arterial oxygen content (Ca_O2) from 10.8 to 7.4 vol/100 vol, but did not elevate epinephrine and norepinephrine levels above 10 and 30 nM, respectively. Despite the substantial reduction in blood oxygen status, the B_max and K_D of myocardial cell-surface -adrenoreceptors were unaffected by 6 h of hypoxic exposure. In addition, acute hypoxemia did not increase myocardial sp70 expression. The failure of short-term hypoxia to decrease trout myocardial -adrenoreceptor density clearly contrasts with the established hypoxia-mediated downregulation shown for mammals. To further investigate the influence of low PO₂ on salmonid myocardial -adrenoreceptors, binding studies were performed on the spongy (continuously exposed to deoxygenated venous blood) and compact (perfused by oxygenated blood supplied by the coronary artery) myocardia of chinook salmon. The spongy myocardium has adapted to its microenvironment of continuous low PO₂ by having 14% more cell-surface -adrenoreceptors compared with the compact myocardium. There was no tissue-specific difference in K_D and no evidence of sexual dimorphism in B_max or K_D. We conclude from our studies that the salmonid heart is well adapted for sustained performance under hypoxic conditions. We found that wild chinook salmon had 2.8× more cell-surface -adrenoreceptors compared with hatchery-reared rainbow trout. This difference suggests a significant degree of plasticity exists for fish myocardial -adrenoreceptors. The signals underlying such differences await further study, but are not likely to include moderate hypoxia and sexual dimorphism.