1. The ionic mechanisms underlying the simple spike action potential in longitudinal myometrium of pregnant rats and the complex action potential which occurs in the same layer of pregnant guinea-pigs are discussed. 2. The current during the upstroke of the simple spike is carried by calcium and repolarization results from inactivation of the calcium current and activation of a potassium current. 3. A slow inward current underlies the plateau component of the complex action potential and calcium is involved in carrying or regulating this current. 4. Single channel recordings from the longitudinal myometrium of pregnant guinea-pigs reveal large conductance (130-170 pS) potassium channels which are activated by depolarization of the membrane. The activation of these channels during the upstroke of the spike would contribute to the rapid termination of the spike. 5. The duration of the plateau component of the complex action potential closely correlates with the duration of contraction and it is suggested that sufficient calcium may enter the cell during the action potential to activate the contractile apparatus directly.