Tubular development continues after birth in full and pre-term infants. As the survival of premature infants increases, serious imbalances in water and electrolytes in this group have become more prevalent. A diminished ability of the immature kidney to reabsorb water and respond to mineralocorticoids, a high excretion of filtered sodium, perinatal complications affecting tubular function, and the use of medications such as diuretics, indomethacin and amphotericin B, are common factors leading to sodium and potassium imbalances in this age group. Appropriate diagnosis and treatment should be guided by a careful assessment of volume status, urine electrolytes and osmolality.