Exercise transiently increases blood viscosity: however data on red cell deformability in this process remain inconsistent, since studies report either impairment (proportional to blood lactate accumulation), a lack of effect, or even in some cases an improvement. To test whether these inconsistencies may be due to physiological differences among populations studied, we compared the effects of in vitro lactate (2 mM, 4 mM and 10 mM) on erythrocyte rigidity in venous blood drawn at rest in 10 untrained vs 10 aerobically-trained subjects. After adjustment of osmolality and pH and incubation at 37 degrees C during 2 minutes, viscometric measurements were performed at 1000 s-1 with the MT90 (falling ball) viscometer and Dintenfass's 'Tk' was calculated. While at baseline there was no significant difference in Tk between the two groups, it decreased in the aerobically-trained subjects between 2 and 10 mM lactate concentrations (p<0.05) and increased in the untrained group between 2 and 4 mM (p<0.05). Thus, it seems that endurance training influences erythrocyte response to lactate. Lactate impaired erythrocyte deformability in untrained subjects but it (unexpectedly) improved it in trained subjects. This difference may be due to training-induced adaptations in erythrocyte metabolism, possibly including transmembrane transfer via monocarboxylate transporters.