A radioimmunoassay developed for the determination of buprenorphine in human urine samples was examined for its applicability to screening post-race equine urine specimens. The detection limit of the immunoassay in equine urine is 0.1 ng ml-1. Confirmation methods using liquid chromatography with electrochemical detection and gas chromatography-mass spectrometry have detection limits of 0.2 ng ml-1 (unchanged buprenorphine and N-des-alkylbuprenorphine) and 0.1 ng ml-1 (after hydrolysis), respectively. The procedures were validated after in-field administration of 300 mug of buprenorphine to a test horse. The procedures were also applied to screening a large number of random post-race equine urine samples.