The effect of pharmacological doses of steroids on pulmonary vascular shunting (Qs/Qt) and fractional pulmonary capillary perfusion (FPCP) following left lower lobe atelectasis (LLLA) was studied in four groups of dogs. LLLA alone resulted in an increase in Qs/Qt. This was followed by a decrease in the blood flow to that lobe so the increased shunting had been essentially corrected after 2 hr. Methylprednisolone (35 mg per kg) prevented this decrease in shunting which occurs after LLLA but the same dose of methylprednisolone failed to affect the pulmonary vascular hypertensive response to ventilation with 8% O2. It is postulated that this steroid acts to partially desensitize the chemoreceptors of the lung to all types of stimuli.