The site, nature, magnitude, and duration of fluid and electrolyte loss into the small intestine during the acute and recovery phase of human cholera was defined in 27 Indian patients. 11 subjects without cholera served as controls. The marker perfusion technique employed was shown, in preliminary experiments, to measure accurately jejunal and ileal fluid and electrolyte transmucosal transport rates under conditions of cholera diarrhea. Fluid loss into the lumen occurred from jejunal and ileal mucosa. The fluid was isotonic in both regions. Bicarbonate concentration was significantly higher in ileal than jejunal fluid during all phases of the disease. Bicarbonate concentration in both regions was significantly higher in acute cholera than during convalescence. Fluid loss into the intestinal lumen ranged from 0.07 to 10.9 ml/hr per cm. Losses were significantly greater from jejunum than ileum. Net ileal absorption was recorded in five of 10 acute cholera studies. During the acute phase of the disease, net jejunal fluid transport showed a positive correlation with fasting intestinal flow rate and stool output. Stool output was also positively correlated with jejunal fasting intestinal flow rates. Recovery of normal fluid and electrolyte absorptive function was usually complete in both jejunum and ileum by the sixth day after admission. These findings in human cholera validate the animal models of choleraic diarrhea and suggest that similar measurements of small intestinal secretory function in other nonspecific diarrheal diseases using the marker perfusion technique may be rewarding.