Smooth muscle, especially gastrointestinal smooth muscle, spontaneously generates oscillatory electrical activity that can control contractions in time and space by altering excitability. The origin and ionic mechanisms underlying these electrical control activities are still controversial, but they behave as coupled relaxation oscillators and they control muscle excitability. Normally, contractions are produced by the addition, during the depolarized phase of the oscillations, of further depolarization by acetylcholine or other means. Pharmacologists who wish to study drug actions on such muscles must be aware of the possibility that drug effects may be determined by these oscillations and may influence contractions by affecting these oscillations as well as by releasing, mimicking, or inhibiting the effects of nerve mediators or by affecting excitation-contraction coupling. Also the use of simplified organ bath preparations may eliminate or alter these control potentials so that results in vitro may not apply in vivo.