The perennial grass Phalaris tuberosa L. is a valuable component of improved pastures in many areas of south-eastern Australia. Under conditions not yet fully understood, it may produce disease in sheep, either a chronic form, "Phalaris staggers", involving incoordination and disorder of the central nervous system; or an acute form, in which there is sudden collapse followed by rapid death or apparent complete recovery. The nature of the toxin has remained unknown. Hypotheses that it might be fluoroacetate or a related fluoro-compound or a cholinesterase inhibitor have not received experimental support. We wish to report the presence in P. tuberosa of tryptamine alkaloids, the known pharmacological properties of which are such that they may be responsible, at least in part, for the toxic effects of the grass. The two bases consistently present as major constituents have been identified as NN-dimethyltryptamine (I) and 5-methoxy-NN-dimethyltryptamine (II). Bufotenine (III) together with uncharacterized indole derivatives is also present. Assays indicate that air-dried samples normally contain 0.05-0.08% total alkaloid, but that fresh grass may contain more than twice the amount that is present after drying. The total alkaloid from fresh grass contains a higher proportion of bufotenine and a lower proportion of the uncharacterized indole derivatives of high R_F value. Much of this latter material obtained from dried grass may thus be artefact. No large differences have been observed in alkaloid composition or content of samples from supposedly toxic and non-toxic pastures, but further investigation of this point is needed because many of the present assays were conducted on dried grass. Wilkinsons has reported the occurrence of 5-methoxy-N-methyltryptamine (IV) and hordenine (V) in Phalaris arundinacea L. We have examined certain unpalatable strains of this species supplied by Drs. R. M. Moore and J. R. McWilliam (cf.9) and find the major base to be gramine (VI), present in amounts up to 0.3%.