The fate of flurazepam hydrochloride, 7-chloro-1-(2-diethylaminoethyl)-5-(2-fluorophenyl)-1, 3-dihydro-2H-1, 4-benzodiazepin-2-one monohydrochloride, having a sleep-inducing effect, was investigated in rats after oral or intravenous administration of this compound labeled with 14C at the C-5 position. It became evident that orally administered compound was absorbed well and highly distributed in some particular tissues followed by rapid excretion in feces and urine. The radioactivity in blood reached a considerably high level at 30 min, but was much lower than that in some tissues, such as liver, lung, kindney, spleen, and abdominal fat after oral administration. Examination of tissue distribution after intravenous administation also indicated extensive tissue uptake of radioactivity. In both routes of administation, radioactivity in the tissues was rapidly eliminated and most of the radioactivity was excreted during the first day in feces and urine. It was found that bile duct-cannulated rat excreted a large portion of the radioactivity in bile after both routes of administration. This result suggestes that the radioactivity excreted in feces by way of the bile duct. Results of these studies demonstrated rapid and complete absorption of this labeled compound, followd by rapid excretion of radioactivity whtih no apparent accamulation in any tissue.