The effect of 30 min of hypoxia followed by reoxygenation on mechanical function was studied in isolated, arterially perfused, neonatal rabbit and dog hearts. All studies were performed at a perfusion rate of 2.5 ml/g-min, at a pacing rate of 60 beats/min and at 27 degrees C. The muscles were perfused with Krebs-Henseleit solutions equilibrated with 95% O2 and 5% CO2 (control) or 95% N2 and 5% CO2 (hypoxia). In the newborn rabbit and dog, both the developed tension (DT) and the maximal rate of tension development (dT/dtmax+) decreased during the first 3 min of hypoxia and then recovered to values not different from control. The effect of hypoxia on DT and dT/dtmax+ was inversely related to age in both the rabbit and dog. The equations describing the decline in DT and dT/dTmax+ during hypoxia and the recovery during reoxygenation were best expressed by two or three exponentials. Time to peak tension and half time to relaxation decreased during hypoxia and the decrease was also inversely related to age. The fact that the newborn was able to maintain normal mechanical function during hypoxia suggests that the newborn is capable of maintaining normal myocardial ATP levels due to enhanced flux through the glycolytic pathway.