Three groups of "healthy' fullterm infants with hyperbilirubinaemia exposed to continuous phototherapy of different intensities, increased their oral intake of milk formula by 33%, 48% and 52% and 24, 48 and 72 h, respectively, irrespective of the intensity of exposure; this was significantly more than in the controls, who were swaddled. Serum osmolality and electrolyte values demonstrated no significant change from pre-phototherapy values, and were similar to those of the control group except for significantly raised potassium levels at 72 h in 2 of the 3 phototherapy groups. These raised values were, however, not dangerously high, and rapidly reverted to normal with cessation of phototherapy. The osmolality of the milk formula feeds was 272.4 +/- 8.2 (mean +/- SE) mmol/kg, potassium 27.8 +/- 1.4 mmol/l and sodium 16.0 +/- 0.8 mmol/l. Phototherapy even for 72 h does not significantly affect the fluid and electrolyte status of hyperbilirubinaemic infants fed with fairly high solute load milk formulas.