While there is considerable evidence to suggest that lactate and pyruvate transport across the cell membrane is controlled, virtually nothing is known about the mechanisms. To test a prediction that sarcolemmal monocarboxylate transport is mediated by a specific carrier, we have examined the kinetics of pyruvate and lactate uptake into aerobically perfused rat hearts. In preparations depleted of intracellular lactate and pyruvate by a 30-min pre-perfusion in the absence of substrates, various concentrations of [14C]lactate or pyruvate (0.02 Ci/mole), together with [3H]mannitol (50 nCi/ml), were transiently (2 min) infused into the mainstream perfusate immediately above the heart. Uptake was calculated from the difference between the level of 14C-labeled substrate predicted from the extracellular distribution of [3H]mannitol and the actual level (corrected for 14CO2 contamination) measured in successive 20-sec samples of effluent perfusate. Computer optimization analysis of the initially rapid (first 60 sec) uptake rates revealed that monocarboxylate transport is not simply a question of diffusion. On the contrary, the observation of typical saturation kinetics (Vmax 7.7-8.4 mumoles/min per g wet wt.) and cross-inhibition (Ki pyr, 2.3 +/- 0.5 mM; Ki lac, 0.16 +/- 0.02 mM) suggest that transsarcolemmal movement of monocarboxylate may be mediated by a high-affinity lactate (km 3.9 +/- 0.9 mM), low-affinity pyruvate (Km 8.6 +/- 1.1 mM), translocase.