Hordenine [4-(2-dimethylaminoethyl)phenol] is chemically closely related to tyramine and several other compounds normally present in mammalian tissues and body fluids. Hordenine itself, however, has not been reported to be a normal constituent in mammals. Hordenine has earlier been used in both human and veterinary medicine in the treatment of diarrhoea and other intestinal disturbances. Hordenine has been detected in the urine of race horses. To date, the Medication Control Programme of Swedish horseracing has found four urine samples to contain hordenine, mainly in the form of an acid-labile conjugate, in the concentration range 1-43 pg/ml. The analysis was performed by thin-layer chromatography (TLC) and the presence of hordenine was confirmed by gas chromatography-mass spectrometry (GC-MS). This work is part of a larger study in which various horse feeds on the Swedish market and plant materials were analysed for the presence of hordenine. Further, the pharmacokinetics of hordenine in the horse and the effects on cardiorespiratory and blood lactate responses to exercise in the horse were studied. Feeding one horse 733 g of germinating barley, a natural source of hordenine containing 63 pg/g of hordenine, resulted in hordenine concentrations in the urine comparable to those mentioned above. The hordenine content in the germinating barley was analysed both by GC-MS and by the present liquid chromatographic (LC) method. Dried barley seedlings (from the malthouse industry) and parts of reed canary grass, also a natural source of hordenine, were analysed by LC with ultraviolet detection. The present chromatographic study of hordenine also included the corresponding N-desmethyl derivatives, tyramine and N-methyltyramine. These are precursors in the formation of hordenine in barley seedlings and certain other plants, and have been reported to occur in dried barley seedlings from the malthouse industry, as also does N-trimethyltyramine (candicin). Pholedrine [4-(2-methylaminopropyl)phenol] was tested for comparison.