In recent studies of equine stereotypic behaviour, data on levels of cortisol and beta-endorphin (BE) have been limited and sometimes contradictory. The current research aimed to investigate, in a large number of horses, the relationships between these compounds and equine stereotypic behaviour. Plasma and salivary cortisol levels were measured in stereotypic (n=46) and normal horses (n=46) to determine whether a significant difference exists between these two groups. No significant differences were found between the mean plasma or salivary cortisol concentrations of stereotypic and normal horses, indicating that their arousal levels are similar. The correlation between plasma and salivary cortisol concentrations of individual horses (n=66) was also examined. A significant correlation between plasma and salivary cortisol levels was found only in horses with an oral stereotypy (r=0.65; P=0.01), which has implications for the use of salivary cortisol as a cardinal indicator of equine acute physiological stress responses. Additionally, plasma BE levels were compared between horses with an oral stereotypy (n=17) and normal horses (n=17). Mean plasma BE levels did not differ significantly between the two groups. Since endogenous opioids are thought to facilitate stereotypic behaviour, and a hereditary component to stereotypic behaviour has been observed, this may suggest that stereotypic horses have inherited opioid receptors with a greater sensitivity than those of normal horses.