Conventional microelectrode and perforated patch clamp techniques were employed to study the pacemaker mechanisms in enzyme-isolated rabbit sinus node cells. 2 mmol/L Cs had little effect on automatic discharge of the SA node and the slope of diastolic depolarization. Cs blocked both If and slope conductance increase but left Ik largely unaffected. The results suggest that If is not essential for the first stage of automatic activity of the rabbit SA node.