Immunoreactive β-endorphin concentration (ir β-EP) was measured in plasma from horses undergoing a number of potentially stressful activities and clinical situations. Resting plasma concentrations of ir β-EP in 14 adult horses was 22.4 ± 2.8 pmol ml-1. The application of an upper lip twitch resulted in a doubling of plasma ir β-EP concentration after 5 min. Prolonged air transportation resulted in a sustained elevation of plasma ir β-EP concentration compared to values measured at rest on the ground during the same day, but short-term road transport (ie. for 1 hour) did not alter circulating plasma ir β-EP concentration in animals accustomed to traveling in a horse trailer. Horses with severe abdominal visceral pain (eg. colic) showed marked elevations in plasma ir β-EP concentration, which may have contributed to any associated hypotensive shock. In contrast, horses with painful, but chronic lameness, had plasma concentration of ir β-EP similar to those of normal horses. Plasma β-EP concentration was considered to be a useful indicator of stress in the horse, however its role in the amelioration of pain would seem to be equivocal.