Pulmonary function was measured by computerized pneumotacography in 26 preterm-newborn infants without respiratory problems. Mean (+/- SD) for birth weight, gestation and postnatal age were 2,034 +/- 412 g, 34.1 +/- 1.7 weeks, and 2.2 +/- 1.6 days, respectively. An esophageal balloon, a differential transducer and nasal prongs were used to measure pressure and flow. The following parameters were measured in each respiration by the mean squares method: Dynamic compliance (Cdyn), total respiratory resistance (RRt) and work (WRt), respiratory time constant (KTt), rate (f) and maximal inspiratory and expiratory flows (PIF, PEF). Replication of the method was found to be good, as the differences between two determinations were not significant (p less than 0.05). The mean differences were less than 6.26%. The mean (+/- SD) and the 3rd and 97th percentiles for each parameter were the following: f: 52 +/- 7 (39-66) r.p.m., Tv: 6.6 +/- 1.1 (4.5-8.7) ml/kg, Vmin: 342 +/- 82 (128-502) ml/min, Cdin: 3.7 +/- 1.1 (1.5-6) ml/cm, RRt: 83 +/- 47 (37-204) cm/L/sec, TRt: 27 +/- 15 (2-57.1) g/cm/kg, PIF: 2.2 +/- 0.5 (1.1-3.2) L/min and PEF: 1.9 +/- 0.6 (0.7-3) L/min.