Ureteral peristaltic activity begins with the origin of electrical activity at pacemaker sites. These sites are located in the proximal portion of the urinary collecting system. The `atypical' smooth muscle cells at these sites fire `pacemaker' potentials at a frequency higher than the `driven' action potentials recorded from typical smooth muscle cells. In contrast to typical smooth muscle cells, these atypical pacemaker cells have less than 40% of their cellular area occupied by contractile filaments and demonstrate a sparse immunoreactivity for alpha-smooth muscle actin. Expression of c-Kit, a tyrosine kinase receptor, correlates with the onset of organized ureteral peristalsis in the embryo. Capsaicin-sensitive sensory afferents and the endogenous release of tachykinins and prostaglandins are involved in the maintenance of normal ureteral peristalsis.