It has been reported that an injection of anti-sheep platelet serum (AsPS) induces transient pulmonary hypertension in sheep, but the mechanism by which this occurs has not been well explained. To examine the hypothesis that pulmonary intravascular macrophages (PIMs) are involved in this phenomenon, we investigated the morphological features of PIMs from sheep that received three different kinds of AsPS injections. The pulmonary arterial pressure response was examined not only in sheep that have many PIMs in the lung and had received AsPS, but also in rats that reportedly have few PIMs and had received an injection of anti-rat platelet serum. The pulmonary arterial pressure responses of the two species were quite different in quality and quantity. In sheep whose pressure response was definitely positive in association with production of thromboxane in the pulmonary circulation, PIMs were found by fluorescent microscopy and electron microscopy to phagocytize aggregated platelets that bound AsPS. These results suggest that PIMs significantly contribute to AsPS-induced transient pulmonary hypertension in sheep.