Pneumocystis carinii (P.c.) pneumonia was induced in 40 rats by a prolonged corticosteroid treatment (group 1); 40 healthy rats of equal weight constituted the control group (group 2); 9 rats received the same corticosteroid treatment as group 1, together with trimethoprim-sulfamethoxazole (TMP-SFZ) in order to prevent P.c. multiplication (group 3). We could distinguish the respiratory effects induced by corticosteroids from those caused by P.c. pneumonia (group 3 vs group 1). For six weeks the blood leukocyte count, the weight of the spleen and the thymus and the pulmonary status were monitored. Blood gases and acid-base status were measured in conscious rats. There was no pulmonary oedema. The infected P.c. rats had a low PaCO2 and a slight disturbance of blood oxygenation, exemplified by A-aDO2 of 30 mmHg, compared with 17.5 mmHg in control rats and 17 mmHg in TMP-SFZ treated rats. P.c. infected rats had a lymphocyte depletion induced by corticosteroids. They did not exhibit respiratory distress. P.c. pneumonia alone in rats did not cause frank hypoxemia.