Background: Sildenafil citrate (Viagra) has been widely used in treatment of erectile dysfunction and thereafter, for the treatment of many vascular diseases as pulmonary arterial hypertension, congestive heart failure and Reynolds's disease. Nowadays, it is introduced in the treatment of wide spectrum of pregnancy-associated diseases as placental blood flow and vascular development are critical to fetal growth and development. Aim of the Work: To study the effect of Viagra on the structure of midterm and full term placentas, to investigate for possible teratogenic effects and to find out the role of Viagra in fetal lung perfusion in albino rats. Material and Methods: A total of 24 pregnant albino rats were divided into 4 equal groups, six rats each: control midterm (They received normal saline and were sacrificed on the 13th  gestational day), control full term (They received normal saline and were sacrificed on the 21st  gestational day), treated midterm (Viagra was administered from the start of pregnancy till the 13th  gestational day), and treated full term (Viagra was administered from the start of pregnancy till the 21st  gestational day). The Viagra dose was 2.5mg/ Kg body weight dissolved in 5 ml saline given daily by gastric tube. Placentas and fetuses of all groups were obtained and processed for light and scanning electron microscopes studies. Results: The midterm and full term Sildenafil-treated placentas showed hypervascularity of the basal and labyrinthine zones. Scanning electron microscopy showed presence of congested chorionic villi as well as presence of congested spiral arteries traversing the inter-villous spaces. Multiple sprouts from the spiral arteries were also observed. No apparent external or internal congenital malformations were detected in full term fetuses. Examination of the fetal-lungs of the treated groups revealed apparent increase in the diameter and the branching of the bronchi and bronchioles, decrease in the interstitial stroma with dilatation and congestion of the interstitial vessels. Conclusion: Sildenafil citrate proved to increase the placental blood flow accompanied with increase in fetal lung perfusion without any apparent fetal teratogenicity