The effects of prostaglandins (PG) B2 and B1 on the pulmonary circulation were studied in the intact spontaneously breathing dog under conditions of controlled blood flow using right heart and transseptal catheterization techniques to isolate and perfuse the left lower lung lobe. Infusion of PGB2 and PGB1 into the left lower lung lobe increased lobar arterial and lobar venous pressure but had no significant effect on left atrial pressure. PGB2 and PGB1 increased pressure gradients between the lobar artery and lobar small vein and between the lobar small vein and the left atrium. PGB2 and PGB1 increased isometric tension in isolated helical segments of lobar vein (3-5 mm diameter) but had little or no effect on lobar arterial segments of the same size. These results indicate that in the intact dog prostaglandins of the B series increase pulmonary vascular resistance by constricting lobar veins and to a lesser extent vessels upstream to lobar small veins, presumably small lobar arteries. PGB2 and PGB1 both produced large increases in pulmonary vascular resistance in the dog with PGB2 being about 10 times as potent as PGB1. It is concluded that PGB2 is one of the most potent pressor substances in the canine pulmonary vascular bed.