Little is known about the role of circulating platelets in PMA-induced lung injury in vivo. We investigated the effects of platelet depletion (PD) on the injury using seven unanesthetized sheep with lung lymph fistulas and eight other sheep for morphological study. PD diminished a decrease in the lymph to plasma concentration ratio (L/P) after 1 microgram/kg PMA treatment (n = 4) and caused more increases in lung lymph flow, L/P and lung lymph clearance after 5 micrograms/kg PMA treatment (n = 3) than in control sheep. The high dose was lethal to platelet-depleted sheep. However, PD had no effects on pulmonary hemodynamics. Morphologically, alveolar hemorrhages and exudate, and bleb formation of type I epithelial cells were more prominent in the platelet-depleted sheep than in the control sheep. We conclude that circulating platelets have protective effects against PMA-induced lung injury but have little involvement in PMA-induced pulmonary hypertension.