Preparations of the herb, goldenseal, have been used in an attempt to thwart detection of morphine in urine of human addicts and racing horses. To assess the potential of goldenseal to interfere with equine drug detection, horses were dosed with morphine ( mg/kg IV) and, in one experimental series, also with goldenseal (60 mg/kg, .). Goldenseal resulted in significant increases in urine volume, specific gravity and acidity. A significant decrease in total morphine excreted occurred 5–6 hours after dosing. It was concluded that with inclusion of an efficient hydrolysis step in the analysis, goldenseal was unlikely to interfere with urine tests for morphine, even if water loading was included in the dosing regimen.