Abstract: The action of platelet activating factor in the rat brain was detected by Evans bluestaining after injection of PAF into the rat brain. The results show that PAF increased the Evans bluestaining of the brain, but no staining was observed without prior injection of PAF. Meanwhile, PAFwas shown to stimulate the release of ¹⁴C-arachidonic acid (¹⁴C--AA) in the cerebral microvascularsmooth muscle cells (CMSMC). SZ-1, a new synthetic drug, dose-dependently inhibited the Evansblue staining of the rat brain and the PAF induced ¹⁴C-AA release in CMSMC. These results indicate that the action of PAF in the rat brain might be related to the stimulation of AA release. SZ-1 may an-tagonize the PAF receptor and protect the brain from PAF induced damage.