Rhythmic electrical activity is a feature of most smooth muscles but the contractile manifestation of this can vary from regular rapid phasic contractions to sustained contracture. The former contractile activity is well suited to fluid propulsion (as in the ureter or lymphatic vessels) while the latter is more suited to providing a variable resistance to flow (as in arteries and arterioles) or to the prevention of flow when this is undesirable (as in the urethra). For many years it was thought that spontaneous electrical activity originated in smooth muscle cells but recently it has become apparent that there are specialized pacemaker cells in many organs that are morphologically and functionally distinct from the surrounding smooth muscle and that the former cells are the source of the spontaneous electrical activity which drives the, otherwise quiescent, smooth muscle cells. We have recently shown that there are specialized pacemaking cells in the rabbit urethra which are similar in gross morphology and ultrastructure to the Interstitial Cells of Cajal found in the gastrointestinal tract (Rumessen & Thuneberg, 1996; Sanders, 1996). The urethral pacemaker cells are excitable, non-contractile, and contain abundant vimentin but no myosin filaments (Sergeant et al. 2000). They have an abundance of calcium-activated chloride current, exhibit regular spontaneous depolarizations which are increased in frequency by noradrenaline and blocked by perfusion with low calcium solution and by chloride channel blockers. The urethral smooth muscle cells, by contrast, are electrically quiescent and have very little calcium-activated chloride current. Electrical activity in the pacemaker cells consists in the regular firing of spontaneous transient depolarizations similar to those observed by Van Helden (1991) in veins. When interstitial cells were voltage clamped at -60 mV spontaneous transient inward currents (STICS) were observed. STICS reversed at ECl, were blocked by anthracene-9-carboxycylic acid as well as niflumic acid and were abolished by perfusion with calcium free Hanks solution suggesting that they were due to rhythmic activation of ICl(Ca). Under suitable recording conditions spontaneous transient outward currents (STOCS) could be recorded in the same preparation as STICS. Both currents were abolished by cyclopiazonic acid, caffeine, or ryanodine, suggesting that they were activated by intracellular Ca²⁺ release. When D-myo-inositol 1,4,5-trisphosphate (IP₃)-sensitive stores were blocked with 2-aminoethoxydiphenyl borate (2-APB), the STICs were abolished but, interestingly, STOCs were not. When measurements were made of intracellular calcium levels in fluo-4-loaded interstitial cells (Johnston et al. 2005) regular calcium oscillations corresponding to spontaneous transient inward currents were observed. Interference with IP₃-induced calcium release using 100 μM 2-APB, or the phospholipase C inhibitors NCDC or U73122 decreased the amplitude of spontaneous oscillations but did not abolish them. However, oscillations were abolished when RyR were blocked with tetracaine or ryanodine. Oscillations ceased in the absence of external calcium and frequency was directly proportional to external calcium concentration. Frequency of calcium oscillation was reduced by SKF96365 but not by nifedipine. Lanthanum and cadmium completely blocked oscillations. We conclude that spontaneous depolarizations in isolated rabbit urethral interstitial cells depend on global calcium waves caused initially by calcium release from ryanodine sensitive intracellular stores. These primary oscillations must be converted to a propagated calcium wave by IP₃-induced calcium release.