In a search for novel platelet-activating factor (PAF) antagonists, we found that 1-(3-phenylpropyl)-4-[2-(3-pyridyl)thiazolidine-4-carboyl]piperazine (3%) showed in vitro and in vivo PAF-antagonistic activities. Introduction of functional groups at the benzylic methylene carbon of 3x afforded some compounds with more potent PAF-antagonistic activity than 3x. Among them 1-(3-methyl-3-phenylbutyl)-4-[2-(3-pyridyl)thiazolidine-4-carbonyl]-piperazine fumarate (3al, YM264) was found to be one of the most potent PAF antagonists.