Hordenine is an alkaloid occurring naturally in grains, sprouting barley, and certain grasses. It is occasionally found in post race urine samples, and therefore we investigated its pharmacological actions in the horse. Hordenine (2.0 mg/kg bodyweight [bwt]) was administered by rapid intravenous (iv) injection to 10 horses. Typically, dosed horses showed a flehmen response and defecated within 60 secs. All horses showed substantial respiratory distress. Respiratory rates increased about 250 per cent and heart rates were approximately double that of resting values. All animals broke out in a sweat shortly after iv injection, but basal body temperature was not affected. These effects were transient, and the animals appeared normal within 30 mins of dosing. Treated horses were tested in a variable interval responding apparatus 30 mins after dosing and no residual stimulation or depressant effects of hordenine were apparent. Animals dosed orally with 2.0 mg/kg bwt of hordenine showed no changes in heart rate, respiratory rate, basal body temperature or behaviour. After iv injection of hordenine, (2.0 mg/kg bwt) plasma reached a maximum value of about 1.0 micrograms/ml, and declined thereafter in a biexponential fashion. Kinetics of plasma concentration satisfied the concept of a two compartment open system, with an alpha-phase half-life of about 3 mins, and a beta-phase half-life of about 35 mins. Total urinary concentrations of hordenine (free and conjugated) peaked at about 400 micrograms/ml, and then declined exponentially to background levels by 24 h after dosing.(ABSTRACT TRUNCATED AT 250 WORDS)