Paxilline was isolated from Penicillium paxilli (NRRL 6110). It was studied together with penitrem B and verruculogen in the electrically stimulated guinea pig ileum. All three mycotoxins enhanced the electrically induced twitch contractions, without influencing the contractions provoked by exogenous acetylcholine. The effect of the mycotoxins could be greatly diminished by hyoscine. The possible mechanism of action of these substances in this in vitro model is discussed. The electrically stimulated guinea pig ileum could be useful in the detection and estimation of the biological activity of tremorgenic mycotoxins.