The failure of clearance mechanisms in the mare’s uterus results in persistent inflammation and is considered a major cause of subfertility. Eighteen mares, of which three were susceptible to endometritis and four had been ovariectomised, underwent charcoal clearance testing to evaluate their clearance mechanisms. This consisted of installing 500 mg of charcoal (particle size 4 to 90 μm) added to 50 ml of phosphate-buffered saline (PBS) into the uterus. Forty-eight hours later the uterus was flushed out with 0·0012 per cent methylene blue in 50 ml of PBS for determination of the diluting factor by photospectrometry. Flush volume, pH, specific gravity and pellet size were all analysed. To investigate the effect of a β2-adrenergic agonist on the ability of genitally healthy oestrous mares to eliminate a suspension of charcoal from the uterus, four genitally healthy mares with appropriate charcoal clearance were also subjected to clearance testing following intravenous administration of 0·8 μg/kg of clenbuterol every 12 hours and 1 μg/kg of clenbuterol every eight hours. All parameters were within their normal range following clenbuterol treatment. However, minimal but significant differences in pre- and post-treatment values regarding fluid volume and extinction rate were recorded.