Interstitial cells from the guinea-pig bladder have been reported to display spontaneous changes in intracellular Ca2+ (1, 2). This signalling has not yet been characterised and its functional significance is unknown. The aim of the present investigation was to study spontaneous Ca2+-activitiy in isolated bladder interstitial cells. Bladders were removed from guinea-pigs which had been killed by cervical dislocation in accordance with UK Home Office regulations. Interstitial cells were enzymatically dispersed from small pieces of detrusor as previously described (1), loaded with fluo 4AM and imaged with a confocal microscope. Cells were washed with Hanks solution for 30 minutes to remove excess indicator before the commencement of recordings. The majority of IC did not display large, regular changes in intracellular Ca2+, however, spontaneous activity was seen in an estimated 10% of cells. Ca2+-signals had mean half-width durations of 8.8±2.9s (data from 7 cells), however the range was from 2.8s to 23.9s indicating the variability of the events. IC fired Ca2+-transients at the rate of 10.5±4.2 per minute and the mean amplitude, F/F0 was 0.73±0.2 (n=7 cells). The spontaneous activity persisted in the presence of xestospongin C, a blocker of IP3 receptors. Application of 10µM carbachol caused a large increase in intracellular Ca2+ and prevented generation of spontaneous signals (n=6). Normal activity returned on washout of the agonist. The findings of the present investigation suggest that bladder interstitial cells exhibit spontaneous Ca2+ signalling and that this activity is affected by cholinergic stimulation.