OBJECTIVE Nonoliguric hyperkalemia has been reported to occur in the first week of life in as many as 50% of extremely low birth weight (ELBW) infants. We studied potassium balance and renal function in the first 5 days of life to characterize potassium metabolism during the three phases of fluid and electrolyte homeostasis that we have described in ELBW infants and to elucidate the factors that contribute to the development of nonoliguric hyperkalemia. METHODS Plasma potassium concentration (PK), potassium intake and output, and renal clearances were obtained for the first 6 days of life in 31 infants with a birth weight of 1000 gm or less. Collection periods in which urine flow rate was greater than or equal to 3 ml/kg per hour and weight loss was greater than or equal to 0.8 gm/kg per hour were denoted to be diuretic. Prediuresis includes all collection periods before the first diuretic period; diuresis includes all collection periods between the first and last diuretic periods; postdiuresis includes all collection periods after the last diuretic period. Infants with a PK greater than 6.7 mmol/L on at least one measurement were denoted to have hyperkalemia. RESULTS PK increased initially after birth--despite the absence of potassium intake- and then decreased and stabilized by the fourth day of life. Diuresis occurred in 27 of 31 infants. The age at which PK peaked was closely related to the onset of diuresis. PK decreased significantly during diuresis as the result of a more negative potassium balance, despite a significant increase in potassium intake. In fact, PK fell to less than 4 mmol/L in 13 of 27 infants during diuresis. After the cessation of diuresis, potassium excretion decreased even though there was a significant increase in potassium intake, potassium balance was zero, and PK stabilized. Hyperkalemia developed in 11 of 31 infants. The pattern of change in PK with age was similar in infants with normokalemia and hyperkalemia: PK initially increased (essentially in the absence of potassium intake) and then decreased and stabilized by the fourth day of life. However, the rise in PK after birth was greater in infants with hyperkalemia than in those with normokalemia: 0.7 +/- 0.2 versus 1.8 +/- 0.2 mmol/L (p < 0.001). No differences in fluid and electrolyte homeostasis or renal function were identified as associated with hyperkalemia. CONCLUSIONS PK increases in most ELBW infants in the first few days after birth as a result of a shift of potassium from the intracellular to the extracellular compartment. The increase in the glomerular filtration rate and in the fractional excretion of sodium, with the onset of diuresis, facilitates potassium excretion, and PK almost invariably decreases. Hyperkalemia seems to be principally the result of a greater intracellular to extracellular potassium shift immediately after birth in some ELBW infants.