The Lipid perturbation hypothesis of the mechanism of psychoactivity of Δ9-tetrahydrocannabinol (THC) is already established and the morphological alteration of rat brain synaptosomal membrane induced by high THC concentration has been reported. But how the change in lipid ordering is related to the morphological change is not yet fully realised. Using an excimer-forming fluorescent probe, pyrene as the reporter molecule, we have studied for the first time, the effect of high dose of this drug on the lipid ordering of rat brain synaptosomal membrane whose THC absorption tendency is very high. Our result shows that below the drug concentration of 1.75 í— 10-2 mg/mg of protein the membrane is less ordered and above this concentration at 3.5 í— 10-2 mg/mg of protein it is more ordered. This indicates that change in lipid ordering is the preincidence of the morphological-damage of synaptosomal membrane in the molecular level and the drug concentration needed for the alteration of membrane ordering is much lower than that needed for the microscopic morphological alteration.