Constipation is a common condition affecting millions of people throughout the world. The present study aimed to determine the effect of extra fluid intake, as recommended by many primary care physicians and gastroenterologists, on the actual stool output in normal healthy volunteers. We recruited 15 healthy volunteers (aged 23-46 years, mean 30.1) without any significant history of diarrhea or constipation to participate in our study. Nine subjects underwent extra intake of isotonic fluids (Gatorade), whereas the remainder received extra free water over their baseline. During period I (3 days), baseline diet and fluid intake were determined by a registered dietitian. During periods II and III (2 days each), the volunteers in each group increased their fluid intake by 1 and 2 1 of isotonic (Gatorade) and hypotonic solution (water), respectively. Period IV (2 days) completed the study with the volunteers returning to their baseline fluid intake. Urine and stool outputs were measured in these volunteers. Additional increase in fluid intake (isotonic or free water) did not result in a significant change in stool output. However, there was a significant increase in urine output (P< 0.05). Despite common medical advice to consume extra fluid for constipation, our results indicate that extra fluid intake in normal healthy volunteers did not produce a significant increase in stool output.