Exogenous surfactant administration improves respiratory function. The speed of improvement appears greater if a natural rather than an artificial surfactant is used, our aim was to determine if that effect was explained by differences in fluid balance, evidenced by the timing of the diuresis onset (i.e. output greater than input). Thirty infants (median gestational age 29 weeks), 10 given an artificial surfactant (Exosurf), 10 a natural surfactant (Survanta) and 10 no surfactant (controls) were studied. During the first three days, compliance and functional residual capacity were measured daily, arginine vasopressin (AVP) levels estimated on days 1, 3 and 5 and, in 8-hourly intervals, the median arterial/alveolar ratio was calculated for each individual and urine output and fluid input recorded. Throughout the three-day period, the median arterial/alveolar ratio was always significantly higher in the control compared to the two surfactant groups (p < 0.05). On day 3 the Exosurf-treated babies had lower compliance and functional residual capacity (p < 0.05) than the other two groups. Neither the timing of diuresis onset, timing of the maximum diuresis nor the AVP levels, however, differed significantly between the groups. Only surfactant treatment and type of surfactant, but not the timing of the onset nor of the maximum diuresis, related significantly to changes in lung function. These results do not support the hypothesis that differences in fluid balance explain differences in the lung function improvement rate following natural and artificial surfactant.