Following surfactant instillation in infants treated for respiratory distress syndrome, a mean arterial blood pressure (MABP) decrease is often observed. Its etiology and pathogenesis are still unknown. In this study various circulatory parameters were recorded continuously after surfactant instillation to elucidate the role of pulmonary vascular resistance as one possible cause for the MABP drop. Seven anesthetized adult New Zealand white rabbits were artificially ventilated after tracheotomy. Arterial and right atrial pressure were recorded continuously. Pulmonary artery pressure and cardiac output were determined by means of a thermodilution catheter. After inducing surfactant deficiency by repeated saline lavages, 200 mg/kg body weight of a natural surfactant preparation was administered by tracheal bolus instillation. PaO2 increased rapidly from 8.0 +/- 1.3 kPa to 51.2 +/- 8.8 kPa (mean +/- standard deviation) within 2 min (p < .05). MABP dropped from 12.1 +/- 1.9 kPa to 8.9 +/- 2.3 kPa within 2 min (p < .05). Pulmonary artery pressure, cardiac output, and right atrial pressure did not change during the observation period of 60 min. The results suggest that a peripheral vasodilatation is the most likely cause for the drop in MABP.