Glomerular filtration rate (GFR) was studied in twelve newborns by means of a single inulin injection. Five of them were of 34 to 35 weeks of gestation (preterm group); the other seven were born between 38 to 41 weeks of pregnancy (term group). Age at time of study varied between three to nine days; weights were normal for gestational age and there was no history of abnormal pregnancy. Inulin was determined according to Schriner's technic and GFR was obtained by Sapirstein's method. GFR was 1.48 +/- 0.70 ml/min in the preterm and 2.40 +/- 0.53 ml/min in the term group less than0.05). However, when body surface was taken into consideration, in the former GFR was 17.28 +/- 8.06 ml/min/1.73 m2 and in the term group 20.64 +/- 4.46 ml/min/1.73 m2; in this way, no significant difference was shown.