Transport of lactate across the erythrocyte membrane proceeds by three distinct pathways: 1) nonionic diffusion of lactic acid, 2) inorganic anion exchange (band 3), and 3) a monocarboxylate-specific (MC) carrier mechanism. This study determined the contributions of these three pathways in the red blood cells (RBCs) of "athletic" and "nonathletic" species. Blood samples were obtained from four male animals of each species: 1) Canis familiaris (dogs), 2) Capra hircus (goats), 3) Equus caballus (horses), and 4) Bos taurus (cattle). Contribution of each pathway to total lactate influx was determined by measuring L-[14C]lactate influx into lactate-depleted control RBCs, p-chloromercuribenzenesulfonic acid (PCMBS)-treated (1 mM) RBCs, and 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS)-treated (0.2 mM) RBCs at three lactate concentrations ([La] values 1.6, 8.1, and 41 mM). PCMBS blocked MC transport and DIDS blocked the band 3 pathway. Lactate influx into the RBCs of the athletic species was 4-160 times faster (P < 0.05) than influx into the RBCs of the nonathletic species at 8.1 and 41 mM [La] values. Nonionic diffusion was greater in the RBCs of nonathletic animals (approximately 7-25%) than in the RBCs of athletic animals (approximately 4%). A significantly higher percentage of the total lactate influx occurred via the band 3 system in the RBCs from the nonathletic animals (approximately 56-83%) vs. the RBCs from the athletic animals (approximately 6-7%) at all [La] values.(ABSTRACT TRUNCATED AT 250 WORDS)