Aim-To examine the eVect of intermittent positive pressure ventilation (IPPV) on plasma arginine vasopressin concentration (pAVP) in preterm neonates. Methods-Thirty five neonates were classified, at the time of blood sampling, into three groups: unstable ventilated; stable ventilated; and stable non-ventilated. A modification of an extraction method for pAVP was developed for use in studies on very small babies, and sampling methods were compared. Results -The pAVP (median, range) was similar in the ventilated (1.85 pmol/l, 0.5 to 3.4) and non-ventilated (2.0, 0.5 to 2.6) stable babies, but was significantly higher (5.7, 1.1 to 25) in the unstable group. There was an inverse correlation between systolic blood pressure and pAVP concentration. Conclusions-This study shows that in preterm neonates pAVP concentration is aVected by the clinical condition and blood pressure, but not by treatment with IPPV.