A transition from constant light (LL) to constant darkness (DD) will reset the circadian rhythms of most organisms to a phase that normally occurs near dusk. We tested the circadian oscillator in the Aplysia eye for this property. The test was run in vitro under two conditions. Using the two eyes of a single animal, one eye was left attached to the cerebral ganglion by its intact optic nerve and the other was detached by cutting its nerve. The amount of LL preceding LL/DD was the independent variable; the phase of the two eyes was the measured variable. When LL less than 12 hr, neither the cut nor the attached eye was reset by LL/DD. When 12 less than or equal to LL less than or equal to 24 hr, the attached eye was reset but the detached eye was not. Thus, for these durations of LL, resetting by LL/DD was found to be dependent on the integrity of optic nerve. When LL was about 21 hr, the differential response of the two eyes caused them to be about 12 hr out of phase. When LL was between 27 and 75 hr, LL/DD reset both eyes but there was a low amplitude cyclic modulation of about +/- 3 hr in the phase to which they were reset by LL/DD. This modulation shows that LL did not stop the eye clock but rather that LL/DD reset its phase while it was free running in LL.