Of 33 infants with hypernatremic dehydration (serum Na+ of greater than or equal to 150 mEq/L) 7 were excluded, 6 because severe alteration of the level of consciousness or shock precluded oral rehydration and 1 because he was given glucose-electrolyte solution plus water. We studied the remaining 27 infants. Twenty (group A) were treated with the World Health Organization-recommended oral rehydration solution (90 mEq/L Na+) and seven (group B) were treated with Pedialyte-RS (Abbott Laboratories Ltd.; 75 mEq/L Na+). The rehydrating solutions were administered in a volume equivalent to twice the clinically estimated fluid deficit. Initial serum sodium was 156.7 +/- 0.9 mEq/L for group A and 155.8 +/- 1.8 mEq/L for group B (mean +/- SEM). The mean time to achieve rehydration was 14.3 and 16.6 h for groups A and B, respectively. Twenty-four hours after commencing oral rehydration, serum Na+ had decreased to 144.8 +/- 1.8 mEq/L for group A and 144.5 +/- 0.9 mEq/L for group B. In two patients in group A, the serum Na+, which, had not decreased to less than 150 mEq/L at 24 h, did so at 48 h. Only in one case (group A) did the serum Na+ increase. This patient had high stool output and failed to become rehydrated after 24 h of unsuccessful oral rehydration. None of the patients had seizures or persistent CNS dysfunction. We conclude that the slow administration of oral rehydration solutions containing either 90 or 75 mEq/L Na+ is a safe and effective treatment of hypernatremic dehydration.