The antifeedant activity towards larvae of Leptinotarsa decemlineata and Spodoptera littoralis and the hot taste for the human tongue have been determined for natural and synthetic 1,4-dialdehydes. Among the bicyclic dialdehydes the biological activity has been found to be dependent on the distance between the two aldehyde groups; tricyclic and pentacyclic dialdehydes are inactive in both tests. A short synthetic route to (-)- and (+)-polygodial is reported.