We examined the effect of anaphylactic shock on the longitudinal distribution of pulmonary vascular resistance (PVR) in ragweed-sensitized dogs in which PVR was partitioned into an upstream arterial component (Rus) and a downstream venous and capillary component (Rds). We also assessed whether Rus and Rds would be reduced by pretreatment with histamine H₁- and H₂-receptor blocking agents and with cyclooxygenase and lipoxygenase pathway inhibitors. Anesthetized animals were examined on separate occasions 3 wk apart in which one of the treatments was randomly given. The pulmonary arterial occlusion technique was used to determine segmental pressure drops. During ragweed challenge, PVR increased 4 times compared with the preshock value (3.04 vs. 12.07 mmHg · l¹ · min;P < 0.05). Although both Rus and Rds increased postshock, the greatest relative increase occurred in Rds. None of the treatments reduced partitioned resistances compared with no treatment. Our results show that, under conditions of anaphylactic shock, increases in Rus and Rds could not be ascribed to release of histamine or products of the cyclooxygenase and lipoxygenase pathways.