Severe electrolyte disorders occur frequently associated with diarrheal dehydration in infants. With this objective we evaluated the erythrocyte activity of Na,K-ATPase in 9 infants dehydrated (D) by diarrhea, on the first and seventh day of recovery. A control group, constituted by 5 normal infants(CI) in addition to another group of 6 normal children (2 to 6 years old-CC) were compared. Na,K-ATPase activity of D was elevated in the first(540±64) and in the seventh day (435±45 nMPi/mg p/h) compared with controls (220±22nMPi/mgp/h). In the D, hyperchloremic metabolic acidosis was associated with normal serum Na and K concentrations. In controls, Na,K-ATPase activity increased with age (r=0,73, p<0,01) and showed an inverse correlation with serum K (r=0,65, p<0,01). Plasma aldosterone concentration was significantly lower in CC(18±4,5,p<0,01) than Cl (73 ± 13 ng/dl). There was an inverse linear correlation between (Na) y and Na,K-ATPase activity (r=0,73; p<0,01) and a positive correlation between the amount of Na administered and Na,K-ATPase activity (r=0,93; p<0,01): In conclusions: 1) The maturity of Na,K-ATPase was related to extra renal regulation of K; 2) the lower activity of this group of this pump in infants is not due to aldosterone deficiency; 3) Na,K-ATPase of crythrocytes was activated maintaining an important electrolyte regulation. This activation is probably a consequence to the K deppletion that occurs in diarrhea.