Khellactones of Peucedanum praeruptorum DUUN, including praeruptorins A (=Pd-Ia, 2) and B (=Pd-II, 11), had an antagonistic effect specifically on platelet aggregation induced by platelet activating factor (PAF) among various aggregating agents examined, and represent a new class of PAF antagonists. We examined the effects of twenty compounds on PAF-induced platelet aggregation and on histamine- and leukotriene D₄ (LTD₄)-induced contractions in isolated guinea pig ileum. Compounds 2, (±)-cis-3', 4'-diacetylkhellactone (3), (±)-cis-4'-acetyl-3'-crotonoylkhellactone (5), (±)-cis-4'-acetyl-3'-tetrolylkhellactone (6), (±)-cis-4'-acetyl-3'-tigloylkhellactone (7), (±)-cis-4'-acetyl-3'-(2"-methylbutyryl)khellactone (8), (±)-cis-3', 4'-ditigloylkhellactone (10), and 11 all strongly inhibited PAF-induced platelet aggregation. (±)-cis-4'-Acetyl-3'-(2"-methyl-2"-dodecenoyl)khellactone (9), (±)-cis-4'-ethyl-3'-tigloylkhellactone (13), (±)-cis-4'-ethyl-3'-[N-(2"-triethylammonio)ethylcarbamoyl]khellactone iodide (16), (±)-trans-3', 4'-diacetylkhellactone (18), (±)-trans-4'-acetyl-3'-crotonoylkhellactone (19), (±)-trans-4'-acetyl-3'-valerylkhellactone (20), (±)-trans-4'-acetyl-3'-isovalerylkhellactone (21), and (±)-trans-4'-acetyl-3'-tigloylkhellactone (22) were weakly inhibitory. Most of the compounds exhibited noncompetitive antagonist actions on histamine- and LTD₄-induced contractions. The potencies of the antagonistic effects on histamine action were in the order 7=22≥2=8=10>6=11=13≥5>19=9 and those on LTD₄ action were in the order 6=22=2>10=8>7=9=11≥13. Thus, compounds with potent PAF-antagonistic activities have the following features : cis isomers of khellactone at the C-3' and C-4' positions are more favorable than trans isomers, and the acyl moiety at the C-3' position of khellactone must be of an appropriate molecular size. In the case of histamine- and LTD₄-antagonistic activities, both isomers show similar effects and acyl moieties of appropriate size are required at the C-3' and C-4' positions. These results are of interest in regard to the medicinal uses of Peucedanum species as a herbal drug.