A small tonic contraction could be evoked by 3 mM adenosine triphosphate (ATP) in the longitudinal muscle of pregnant rat myometrium which was incubated with the Ca-, Mg-free solution containing 2 mM EGTA (40 mM K). The amplitude of the tonic contraction was decreased by exposure to the solution within a few hours, but was again increased when 1-10 mM Mg was added to the Ca-free solution. The ATP-induced contraction was increased for more than 7 h during exposure to the Ca-free solution containing Mg ions. The ATP-induced contraction in the Ca-free (Mg-free) solution was also produced when the solution contained 1 mM Mn or 1.5 mM La. On the other hand, the tonic contraction evoked by 1 mM Ca or Ba in the presence of 40 mM K was depressed by ATP. The above results led us to propose that the ATP-induced contraction evoked in the Ca-free solution containing Mg was mediated by a process independent of Ca, and possibly mediated by a translocation of Mg in the cell. La and Mn could take the place of Mg in producing a tonic contraction caused by ATP application.