The locomotor activity of individualMusca domesticaflies was recorded photoelectrically. The pattern consisted of unimodal circadian rhythms in 29% of the cases, of more than one circadian rhythm visible in the same actogram ("compound rhythms") in 63% and was arrhythmic in 8% of the cases. When 1 mM LiC1 was added to the drinking water, only5%of the flies which exhibited an unimodal rhythm before were still showing this pattern. 57% snowed a compound rhythm and 38% became arrhythmic under LiCl. Flies with compound rhythms continued to show this pattern (73%), or became arrhythmic (27%). Arrhythmic flies stayed arrhythmic under Li+treatment. Thus Li+decreases the percentage of rhythmic and increases the percentage of arrhythmic flies. This finding is discussed in respect to reduced mutual coupling of circadian pacemakers and complete asynchrony among the individual oscillators after Li+application.