Reasons for performing the study: L-Tryptophan is a common ingredient in equine calmative products, but its effectiveness has not been demonstrated in horses.Hypothesis or objectives: To determine whether a commercial dose of l'tryptophan increases plasma tryptophan and alters behaviour in horses fed a roughage or concentrate meal.Methods: L-tryptophan (6.3 g) or placebo (water) were administered (p.o.) in a cross over design, to 12 Thoroughbred horses (503 ± 12.1 kg bwt), just before a meal of lucerne hay (study 1) or oats (study 2). Plasma tryptophan was measured by gas chromatography. Horse behaviour was observed in an empty enclosure, then in the presence of an unfamiliar person and a novel object.Results: Total plasma tryptophan increased 3-fold in both studies, peaking 1.5 h to 2 h after dosing. After the peak, tryptophan remained high for several hours if the horses had been fed hay, but fell sharply if the horses had received oats, consistent with the glycaemic responses to these meals. However, the ratio of tryptophan to four large neutral amino acids (phenylalanine, tyrosine, leucine and isoleucine) increased in the tryptophan-treated horses to a similar extent, and for a similar duration, with both diets. The presence of a stranger or novel object increased heart rate (P < 0.05), but caused no behavioural effects that were altered by tryptophan, regardless of the diet.Conclusions: Plasma tryptophan increases when tryptophan is administered at a dose used in some commercial products, but this is not reflected by marked behavioural changes in the horse.Potential relevance: Further work is required to refine behavioural tests and identify an effective dose of l-tryptophan in the horse.