We have previously shown that a mechanism for transporting L-lactate is located in cardiac sarcolemmal membranes (Am. J. Physiol. 252:C483-C489, 1987). This mechanism has now been shown to transport pyruvate also. The transporter recognizes a wide range of monocarboxylic acids with chain lengths of three to six carbons, as evidenced by their ability to inhibit L-lactate uptake into sarcolemmal vesicles. The ability of the monocarboxylate analogues to inhibit depends strongly on the nature of substituents, particularly at the second carbon. L-lactate and pyruvate transport are not affected by dicarboxylates other than oxaloacetate. The transporter is inhibited by the protein modifiers diethylpyrocarbonate, dinitrofluorobenzene, and phenylisothiocyanate. Diethylpyrocarbonate inhibition is not reversed by hydroxylamine, nor is dinitrofluorobenzene inhibition reversed by thiol reagents, suggesting that the target residues are not histidine, or tyrosine or cysteine, respectively. Several monocarboxylates effectively protect the transporter from inhibition by the modifying reagents, suggesting that the modified residue(s) may be at or near the binding site. Alternatively, the target amino acid(s) in the transport protein may become inaccessible due to a conformation change triggered by the substrate analogues. Overall, the results suggest that a sensitive free amino group, associated with substrate binding, is attacked by the protein-modifying reagents.