Two investigation benzodiazepine hypnotics with long half-lifes, (30 and 15 mg quazepam and 30 mg flurazepam) were evaluated in 47-night sleep laboratory studies. The effectiveness and side effects of these benzodiazepines were assessed during short-, intermediate-, and long-term use. Subjects were also assessed for presence of rebound insomnia during the 15 days following abrupt withdrawal. Quazepam, 15 and 30 mg, and flurazepam, 30 mg, each were effective in sleep induction and maintenance after short- and intermediate-term use. Some loss of effectiveness was noted during long-term use of both doses of quazepam and, to a lesser extent, of flurazepam. Subjective reports of improvement in sleep latency and total sleep time were in general agreement with the objective data. During the 15 nights after abrupt withdrawal of these two long-half-life drugs there was no rebound insomnia, either immediate or delayed. Both drugs exerted carry-over effectiveness on the first 2 to 3 nights after withdrawal; with quazepam this effect persisted throughout the withdrawal period. Quazepam, 30 mg, induced frequent side effects related to sleepiness. Side effects noted with 30 mg flurazepam were less frequent and severe, while the side effects with 15 mg quazepam were minimal. These data suggest that the optimal dose of quazepam is 15 mg.