The current experiment examined the effect of switching from a moderate grain diet to ahigh grain diet, as are typically fed to intensively managed horses, on some behaviouraland physiological (oro-caecal transit time [OCTT], digestibility, plasma cortisolconcentrations and heart rate) parameters in adult cribbers (n = 5), weavers (n = 6) andnon-stereotypic Thoroughbred horses (n = 6). The cribbers and weavers in the study hadbeen known to show stereotypic behaviour for at least 12 months prior to commencementof the study. Switching to a high grain diet reduced explorative behaviour (P < 0.01) butdid not affect stereotypic behaviour. Horses took more time to consume the entire highgrain ration than the moderate grain ration (P < 0.01), possibly in an attempt to slow therate of starch delivery to the hindgut. The high grain diet had lower digestibility than themoderate grain diet (P < 0.01). Switching to a high grain diet was accompanied by areduction in water intake (P < 0.05), though the two diets did not differ significantly intheir effects on plasma cortisol concentration, heart rate, or OCCT. Combined, thesefindings suggest that horses may have altered their feeding habits in response to thechange in diet and this change in behaviour could have been an attempt to limit dietinducedacid build-up in the hindgut. Future studies on the relationship between the levelof grain in the diet and stereotypic behaviour should consider that horses may showfeeding changes to different diets which attenuate diet-induced effects on physiology andother behaviour.