Hyperglycemia and glucosuria are frequently seen during the acute phase of dehydration secondary to gastroenteritis. In this paper, 33 patients in hypovolemic shock due to diarrhea were studied. Serum glucose resulted above 140 mg/100 ml. in 14 patients (41%); it was below 27 mg/100 ml. in 2 more patients (5.8%). Despite high glucose concentration, only 4 out of the 14 hyperglycemic patients had serum insulin concentration above that observed in normal fasting children. Plasma cortisol was significantly higher in the group with hyperglycemia (54 +/- 24 micrograms/100 ml.) in comparison with the group with normal glucose levels (15 +/- 6.4 micrograms/100). No correlation was found between serum glucose and the concentrations of sodium, potassium, bicarbonate, growth hormone nor with plasma osmolarity. All disturbances observed turned normal after rehydration. Hyperglicemia is explained as a response to stress and is probably due to the gluconeogenic action of cortisol and the inhibitory effect of catecholamines upon insulin secretion.