A controlled, randomized trial was conducted in 50 infants with acute dehydrating diarrhea to evaluate the effect of oral zinc supplementation in acute diarrhea. After completion of rehydration, 25 infants in Group A received oral zinc sulfate (20 mg elemental zinc twice daily) and an equal number in Group B were given placebo (glucose). Both groups were comparable with respect to various initial characteristics including nutritional status, diarrheal disease, serum alkaline phosphatase, and serum and rectal mucosal zinc content. During therapy all the assessed parameters of zinc status (serum alkaline phosphatase and serum and rectal zinc) recorded significant elevation and reduction in Groups A and B, respectively. At recovery the zinc status of Group A was significantly better and was nearer that of healthy controls. The diarrheal duration and frequency in the zinc-supplemented group were lower, but the differences were not significant (0.05 less than p less than 0.1). However, when only subjects with relatively severe initial zinc depletion (rectal zinc lower than the 15th percentile of healthy controls; 11 in Group A and 14 in Group B) were considered, the diarrheal duration and frequency were significantly (p less than 0.05 and p less than 0.01, respectively) lower in the zinc-supplemented cases. Weight gain in both groups was similar. It is concluded that oral zinc administration in acute diarrhea can replenish body zinc status and this may shorten the diarrheal duration and frequency in children with relatively severe zinc depletion.