Isometrically mounted and electrically paced myocardial venticular strips of carp have a much higher capacity to develop force during severe hypoxia and to redevelop force after it than those of rainbow trout. When the concentrations of CO2 and HCO3- in the solutions surrounding the strips were increased together, such that pH remained constant, the force developed during hypoxia increased. The concentration of CO2 was raised from 0-4%; that of HCO3- from 0-25 mM. The effect was much more pronounced in the carp strips than in the trout strips. With the carp strips the force recovery on reoxygenation was unaffected by the variations in CO2 and HCO3-. The trout strips recovered better when CO2 and HCO3- was raised during either hypoxia or reoxygenation.