The disposition of 14C-loprazolam has been studied in rat, dog, cynomolgus monkey and man using oral and parenteral dosing. In all species 14C was excreted principally in the faeces irrespective of the route of administration. In surgically prepared animals, 46% dose (rat) or 60% (dog) was excreted in bile and, together with urinary excretion, indicates that approx. two-thirds of an oral dose was absorbed. In rat there was relatively little enterohepatic circulation (approximately 26%) compared to dog (approximately 73%). Whole-body autoradiography and tissue-distribution studies in rat showed that 14C was distributed principally in liver and intestine, and was eliminated within 24 h. 14C was found in brain of rat and dog; in dog concn. levels were higher in white matter than in grey matter. In studies using pigmented animals, 14C was associated with the uveal tract of the eye and with other melanin-containing tissues. This was reversible and was eliminated from the eye of rat with a half-life of 3.4 d. Blood and plasma concn. of 14C and of unchanged loprazolam declined relatively rapidly in rat. In dog, cynomolgus monkey and man, total 14C concn. in blood fell more slowly than unchanged loprazolam. Less than 85% of loprazolam was protein bound in rat, dog or human plasma in vitro.