1. Smooth muscle cells from guinea-pig urinary bladder were studied at an extracellular Ca2+ concentration ([Ca2+]o) of 3.6 mM and 36 degrees C. Fluorescence of Indo-1 was used to monitor the cytosolic calcium concentration ([Ca2+]i) and its changes ([Ca2+]i transients) induced by step membrane depolarizations. 2. During a 6 s depolarization step from -60 to 0 mV [Ca2+]i increased from a resting 118 +/- 22 nM to 1150 +/- 336 nM and decayed to a sustained level of 295 +/- 62 nM. The experiments were designed to evaluate the contribution of the release of intracellularly stored Ca2+ to components of the depolarization-induced [Ca2+]i transient, i.e. 'phasic', which decayed during a maintained depolarization step, and 'tonic' which constituted the sustained elevation of [Ca2+]i above resting level. 3. A short (1 s) application of 10 mM caffeine mimicked the phasic component. After wash-out of caffeine, the subsequent depolarization induced a [Ca2+]i transient with reduced peak, the degree of suppression depending on the interval between wash-out of caffeine and depolarization. The phasic component of the depolarization and the caffeine-induced [Ca2+]i transients were not additive but saturative. 4. The phasic component was largely abolished in the continuous presence of 10 mM caffeine. It was also abolished by a 10 min cell dialysis of 10 microM ryanodine from the pipette solution and was strongly reduced by dialysis of 5 microM thapsigargin. Changes of the tonic component of the depolarization-induced [Ca2+]i transient were much less pronounced with all three interventions. 5. The tonic component of the depolarization-induced [Ca2+]i transient was increased when [Ca2+]o was elevated briefly before a depolarization close to 0 mV, whereas the phasic component was not significantly changed. Similarly, brief application of 1 microM Bay K 8644 increased the tonic component several-fold without modifying significantly the phasic component. 6. It is concluded that depolarization-induced influx of Ca2+ through L-type Ca2+ channels induces the release of Ca2+ from intracellular caffeine-sensitive stores which constitutes the major part of the phasic component. Ca2+ release superimposes on the effects of Ca2+ influx through L-type Ca2+ channels, the non-inactivating part of which constitutes the tonic component of the [Ca2+]i transient. Since the two processes interact, a dissection by simple subtraction is not possible.