Pneumocystis carinii is an opportunistic sporozoan parasite which affects the lungs of a variety of mammals and causes fatal diffuse inter­stitial pneumonia, especially in cases of low immunity. Experimental in­fection with P. carinii was induced by immunosuppression of rats that ac­quired the infection naturally after once, with 2 mg/L dexamethasone sodium phosphate with 500 mg/L tetracycline hydrochloride in daily drinking water. Thirty-nine albino rats obtained from laboratory colony (200 - 250 gm each) were used. They were sacrificed 1, 2 and 3 months after starting drug administration. Lungs were macroscopically examined and then, smears were taken and stained with Giemsa and to­luidine blue 0 stains. Examination of lungs of rats sacrificed I month-post drug administration revealed no changes, while, lungs of rats sacrificed 2 and 3 months post-drug administration showed congestion and appear­ance of greyish-white nodules on the lung pleura. Lung smears stained with Giemsa from such rats showed different developmental stages of P. carinii such as the trophozoites, small and large, the precyst and the ma­ture cysts. Lung smears stained with toluidine blue 0 stain showed the cystic stage only. A survey study on 27 rabbits, 3 dogs, 7 sheep, 5 goats. 10 hors­es, 33 cattle and 16 camels was carried out. Lung smears examination of these animals revealed that 4, 2, 3, 1, 4, 3 and 0 were found positive to P. carinii. The zoonotic importance of the parasite was discussed.