We have previously reported that (±)-[6]-and (±)-[8]-gingerols potentiate prostaglandin (PG) F_2α-induced muscle contraction, and that other gingerol-related derivatives do not necessarily produce the same effect. The moiety necessary for potentiation was therefore investigated. 1) (±)-[8]-Gingerol potentiated PGF_2α-induced contraction to a greater extent than (±)-[6]-gingerol and (±)-hexahydrocurcumine (HHC), but [6]-shogaol produced inhibition ; 2) Noradrenaline (NA)-induced contraction was inhibited in the following order : [6]-shogaol>(±)-[8]-gingerol>[6]-gingerdione>(±)-[6]-gingerol>S-(+)-[6]-gingerdiacetate (GDA) >[6]-dehydrogingerdione (DHG), whereas (±)-HHC had no significant inhibitory action ; 3) [6]-gingerdione had different effects on PGF_2α-induced contraction, indicating inhibition just after preparation of the solution, no effect apparent after 2 h, and potentiation after 5 h ; 4) [6]-gingerdione showed a change in its chemical structure after 5 h as measured by the FeCl₃ reaction, and ultraviolet and ¹H nuclear magnetic resonance spectra. It was concluded that the aliphatic hydroxyl group present in gingerol derivatives is necessary for potentiation of PGF_2α-and the keto group for inhibition of NA-induced contraction.