Granier, P., H. Dubouchaud, N. Eydoux, J. Mercier, and C. Préfaut. Effect of 2-chloropropionate on initial lactate uptake by rat skeletal muscle sarcolemmal vesicles. J. Appl. Physiol. 81(5): 1973-1977, 1996.2-Chloropropionate (2-CP) is a halogenated monocarboxylic acid generally used to decrease blood lactate concentration in various metabolic states. To investigate whether it has an inhibitory effect on sarcolemmal lactate transport, we compared the initial rate of lactate transport in sarcolemmal membrane vesicles purified from 20 male Wistar rats with and without 2-CP. Transport by these vesicles was measured as uptake of L-(+)-[U-¹⁴C]lactate under pH gradient-stimulated cis inhibition. The time courses of 1 mM L-(+)-lactate uptake into vesicles both with and without 10 mM 2-CP (L- or D-) displayed saturation kinetics. Lactate uptake values were lower with 10 mM L-2-CP and 10 mM D-2-CP in comparison to the control values. Both 10 mM L-2-CP and 10 mM D-2-CP significantly inhibited 1 mM L-(+)-lactate uptake (55.8 ± 9.1 and 53.5 ± 12.1%, respectively; P < 0.001), whereas a smaller inhibition was observed with a higher lactate concentration of 50 mM (40.2 ± 11.2 and 38.7 ± 12.4%; P < 0.001 and P < 0.05, respectively). However, a higher D-2-CP concentration (50 mM) increased the inhibition of pH-stimulated 1 mM L-(+)-lactate uptake (77.0 ± 9.4%; P < 0.001). D-2-CP had a trans-stimulation effect on the initial rate of lactate efflux of 1 mM L-(+)-lactate compared with baseline efflux (9.5 ± 0.8 vs. 5.1 ± 0.4 nmol ^· min^{1 ·} mg protein¹; P < 0.05). 2-CP significantly inhibited the initial rate of lactate uptake in skeletal muscle sarcolemmal membrane vesicles. This result suggests that 2-CP is a nonstereoselective substrate of the lactate muscle carrier that impairs lactate transport.