In Drosophila melanogaster, the clock gene period (per) has a clearly defined role in the molecular machinery involved in generating free-running circadian rhythms. per mutations also influence rhythms in the Drosophila love song and in the ultradian timescale. The relationship between these two phenomena has so far escaped satisfactory explanation. Here we analyzed the neurophysiological activity of the giant fiber neural pathway in per(0) flies. Under constant light, and at relatively low stimulation frequencies (1-2 Hz), per(01) flies habituate significantly earlier than they do under 12 h light-dark cycles. The results suggest an involvement of per in phenomena of short-term neural plasticity.