There is a large daily fluctuation in the willingness of both males and females of Dacus tryoni to respond with sexual behaviour to a standard light stimulus. The daily fluctuation in sexual drive level persists under constant low light intensity with a period of about 28 hr so it is timed by a circadian clock. Experiments were carried out on the decline in drive level that occurs after each daily peak. Expression of sexual behaviour plays little or no part in causing this decline but periods of low light intenstiy cause large and long lasting reductions in sexual drive level. A consistent relationship was found between the immediate effect of periods of low light intensity on the drive level and their effect on the subsequent phase of the fluctuation (i.e. their effect on the circadian clock). This suggests that the endogenous clock not only controls the timing of fluctuations in drive level but also determines the level of sexual drive at each time of day.