The blood lactic acid content of swimming mice were detected to study the effect of continued time and intensity of movement, and blood sampling time on that. The results indicated that during the movement the variations of lactic acid content was increasing, decreasing and continued increasing with the more strenuous exercises. The content was higher significantly when sampling after long-term movement than sampling at 15 minutes after the movement begin. Author's address Wang Hongmei, the Health and Anti-epidemic Station of Beijing, 100013 PRC.