The six (racemic or meso) isomers of 3,4-dimethyl-2,5-bis(3,4-dimethoxyphenyl)tetrahydrofuran and four corresponding desmethyl analogues were prepared and assayed as inhibitors of platelet activating factor (PAF) receptor binding to rabbit platelet plasma membranes. The inhibition by these isomers is stereodependent and varies with the gross shape of the molecules as determined by the molecular mechanics program MM2. The most potent PAF antagonist in this group of compounds is trans-2,5-bis(3,4,5-trimethoxyphenyl)tetrahydrofuran (L-652,731, 14) with an IC50 of 0.02 microM.