The aim of this study was to investigate the effect of L-lactic acid on swimming endurance of mice. Mice (n = 50) were injected intraperitoneally with saline, then with L-lactic acid (either 25 mg/kg or 50 mg/kg body weight), then after 2 days with the same doses of glucose, and after another 2 days again with L-lactic acid at the same doses. Swimming times to exhaustion were determined at 30 min after each injection, in a tank filled with 25 cm of water maintained at 23 degrees C. After another week, mice were given either saline, L-lactic acid, or glucose (25 or 50 mg/kg) dissolved in saline and sacrificed after 30 min for biochemical analyses. The ratios of swimming times of L-lactic acid or glucose injections to saline injection were calculated as an index for endurance changes. Swimming-time ratios for mice injected with L-lactic acid were significantly higher at either dose than for those injected with the corresponding doses of glucose (p < .05). The ratio of swimming time was greater in those given a dose of 50 mg/kg than in those given 25 mg/kg for mice in the L-lactic acid groups (p < .05) but not in the groups given glucose. There were no marked differences in biochemical parameters of plasma and muscle lactate, muscle and liver glycogen, or plasma glucose and nonesterified fatty acid between the L-lactic acid, glucose, and saline injection groups. These results suggest that L-lactic acid can enhance swimming endurance of mice and that this action is dose dependent.