Infection of perennial ryegrass (Lolium perenne L.) with appropriate strains of the endophytic fungus Acremonium lolii Latch, Christensen et Samuels results in the production of a number of fungal secondary metabolites of which the insect antifeedant peramine and the lolitrem neurotoxins are the best characterised. Peramine, a novel pyrrolopyrazine alkaloid, occurs in many endophyte/grass associations and is a highly active feeding deterrent to the Argentine stem weevil (Listronotus bonariensis Kuschel). The lolitrems are complex indole terpenes associated particularly with the endophyte A. lolii and are considered responsible for the ryegrass staggers toxicity syndrome shown by livestock grazing endophyte-infected pastures. Lolitrem B is tremorgenic to mice and toxic to Argentine stem weevil. Paxilline, a weak tremorgen and presumed biosynthetic precursor of the lolitrems, is also produced by A. lolii. The chemistry and biological activity of peramine, lolitrem and paxilline is reviewed. © 1993.