In precordial pacing with skin-surface electrodes, the goal is to excite the ventricles with minimal stimulation of overlying tissues. A theoretical analysis is presented to identify the relevant factors; the two most important are electrode location and pulse duration. Using the basic law of stimulation, we developed a model which indicates that the optimum pulse duration for the closed-chest pacing pulse is one that is long with respect to the membrane time constant of cardiac muscle. Current-versus-duration curves are presented for pacing and pain, based on experimentally obtained data. For pacing with minimum pain, the optimum stimulus duration was found to be about 10 ms.