To test a prediction that trans-sarcolemmal lactate movement is carrier mediated, 14C-labelled lactate or pyruvate and 3H-mannitol were transiently infused (2 min) into aerobically perfused rat hearts that had been depleted of intracellular monocarboxylates by a 30 min pre-perfusion in the absence of substrates. Uptake was calculated from the difference between the level of 14C-substrate predicted from the extracellular distribution of 3H-mannitol and the actual level (corrected for 14CO2 contamination) measured in successive 20 s samples of effluent perfusate. Computer optimization analysis of the initial influx revealed that monocarboxylate transport is not simply a question of diffusion. On the contrary, saturation and cross-inhibition (Ki, lactate 0.14 +/- 0.04 mM; Ki, pyruvate 2.2 +/- 0.4 mM; Ki, acetoacetate 6.2 +/- 0.9 mM; and Ki, 3-hydroxybutyrate 20 +/- 6.0 mM) suggest that trans-sarcolemmal monocarboxylate movements are reversibly mediated by a high activity (Vmax 34 +/- 7 mumol/min/g wet wt), low affinity lactate (Km 6 +/- 2 mM) permease. Further, studies into the effects of pH indicate that transport requires prior protonation of the carrier (pKa 7.1) and that lactate movements are driven by the displacement of the trans-sarcolemmal lactate and proton gradient from the Donnan equilibrium.