One hundred twenty children below 5 years of age with diarrhea caused by Vibrio cholerae, enterotoxigenic Escherichia coli, or rotavirus were studied for stool electrolyte composition and purging rates. The mean purging rate in cholera was 60.1 ml, in ETEC 39.2 ml, and in rotavirus infection 31.4 ml/kg/8 hour. The mean stool sodium concentration in cholera was 88.9 mMol/L, in ETEC 53.7 mMol/L, and in rotavirus infection 37.2 mMol/L. Stool potassium concentration did not show much variation, Mean CO2 concentration in rotavirus infection was 6 mMol/L, significantly lower than in cholera and in ETEC diarrhea. In cholera, stool sodium concentration increased significantly with increase in purging rates; the same was not true in rotavirus and ETEC diarrhea. These differences are considered important factors in formulating replacement therapy in diarrhea.