The Rats: Exsposcd During Fetal Life %/ Nonsteroidal antiinflammatory drugs which are used during pregnancy can produce constriction of the ductus arteriosus and pulmonary arteries in the fetus. In this study we investigated the effect of diclofenac sodium, a nonsteroidal antiinflammatory drug, on the number of pulmonary capillaries A total of 20 Swiss albino rats, 10 experiments and 10 controls, were used. From the begining of 5th day of gestation, diclofenac sodium was administered to the experiment group in a period of 15 days. In the same period of time, 1 ml physiological saline were given to the control groups. After delivery the rats were fed with normal diet for four weeks and their lungs were taken to prepare slides. Capillaries were counted in the sections. II was observed that the experiment group has 2800 capillaries/mm whereas the control group has 3400 capillaries/mm in the lung. The number of capillaries of the experiment group (per mm2) was decreased than those of controls. This difference between two groups was statistically significant (p