The effect of Li+, Rb+and tetraethylammoniumchloride (TEA) on the locomotor activity rhythm ofMusca domesticawas studied. Li+as well as TEA lengthen the free run period t. This effect was more pronounced in animals with shorter period lengths. The effect of Rb* depends on the previous period length. In flies which had a short period, Rb+lengthened, and in those with a long period Rb+shortened t. Replacing the Li+solution with water reverted the period lengthening partly. In the case of Rb* and TEA the rhythm became obscured after replacing the solutions with water. The involvement of K+ions and transport ATPases in the expression of circadian rhythms is discussed.