We investigated whether a glycolytic burst contributes to the initial adaptation of ATP synthesis to increased cardiac metabolic demand. Six isolated rabbit hearts were perfused with glucose-containing Tyrode solution at 28 degrees C. In venous and arterial samples the lactate concentration was determined with a sensitive enzymatic cycling method. After the heart rate was doubled from 60 to 120 beats/min, lactate efflux increased from 0.23 +/- 0.10 (SE) to 0.45 +/- 0.12 mumol.min-1.g-1 dry weight with a mean response time of 21.3 s but without an overshoot. The transport time for lactate is longer than 15.7 s, suggesting that lactate production adapts with a mean response time of less than 6 s. Because no overshoot in lactate efflux was found, it is unlikely that a glycolytic burst after a step in heart rate contributes to the fast adaptation of ATP synthesis to demand in the isolated rabbit heart, although it might be possible that a change in cytosolic lactate production is not reflected in an increase in lactate efflux. Extrapolation of the results of this study to the in vivo situation should be done with caution.