We tested in six fish species [Pacific lamprey (Lampetra richardsoni), Pacific spiny dogfish (Squalus suckleyi), Asian swamp eel (Monopterus albus), white sturgeon (Acipenser transmontanus), zebrafish (Danio rerio), and starry flounder (Platichthys stellatus)] the hypothesis that elevated extracellular [HCO3-] protects spontaneous heart rate and cardiac force development from the known impairments that severe hypoxia and hypercapnic acidosis can induce. Hearts were exposed in vitro to either severe hypoxia (~ 3% of air saturation), or severe hypercapnic acidosis (either 7.5% CO2 or 15% CO2), which reduced heart rate (in six test species) and net force development (in three test species). During hypoxia, heart rate was restored by [HCO3-] in a dose-dependent fashion in lamprey, dogfish and eel (EC50 = 5, 25 and 30 mM, respectively), but not in sturgeon, zebrafish or flounder. During hypercapnia, elevated [HCO3-] completely restored heart rate in dogfish, eel and sturgeon (EC50 = 5, 25 and 30 mM, respectively), had a partial effect in lamprey and zebrafish, and had no effect in flounder. Elevated [HCO3-], however, had no significant effect on net force of electrically paced ventricular strips from dogfish, eel and flounder during hypoxia and hypercapnia. Only in lamprey hearts did a specific soluble adenylyl cyclase (sAC) inhibitor, KH7, block the HCO3--mediated rescue of heart rate during both hypoxia and hypercapnia, and was the only species where we conclusively demonstrated sAC activity was involved in the protective effects of HCO3- on cardiac function. Our results suggest a common HCO3--dependent, sAC-dependent transduction pathway for heart rate recovery exists in cyclostomes and a HCO3--dependent, sAC-independent pathway exists in other fish species.