Eighteen very low birth weight premature infants born before 28 weeks gestation and weighing less than 1000 gm were evaluated prospectively for disturbances in serum electrolyte concentrations and for renal glomerular and tubular functions. Clinically symptomatic hyperkalemia resulting in significant electrocardiographic dysrhythmias developed in eight of these infants; 10 babies remained normokalemic. Peak serum potassium concentration ranged from 6.9 to 9.2 mEq/L in the hyperkalemic group; all potassium values in the normokalemic group were less than 6.6 mEq/L. Indices of renal glomerular function and urine output were similar in both groups; no infant had oliguria. Serum creatinine concentrations were the same in both groups (1.04 +/- 0.16 SD mg/dl in normokalemic vs 1.19 +/- 0.24 mg/dl in hyperkalemic infants, beta less than 0.2 at alpha = 0.05), and glomerular filtration rates did not differ significantly (6.29 +/- 1.78 ml/min/1.73 m2 in normokalemic vs 5.70 +/- 1.94 ml/min/1.73 m2 in hyperkalemic infants, beta less than 0.2 at alpha = 0.05). In contrast, indicators of tubular function revealed a significantly larger fractional excretion of sodium in hyperkalemic infants: 13.9 +/- 5.4% versus 5.6 +/- 0.9% in normokalemic control subjects (p less than 0.001). Hyperkalemic infants also had a tendency toward lower urine concentrations of potassium, although there was no significant difference in their net potassium excretion in comparison with that in the normokalemic group. We speculate that hyperkalemia in the tiny baby is in part the result of immature distal tubule function with a compromise in ability to regulate potassium balance.