Detrusor overactivity is a highly prevalent disorder severely affecting patient quality of life and is significantly associated with increased spontaneous activity in isolated bladder wall preparations. Therefore reducing spontaneous activity offers a means to relieve detrusor overactivity. Previously, we have shown that mild heating (42°C) of isolated bladder strips reversibly reduced spontaneous activity without tissue damage and we have tested the hypothesis that greater heating to 46°C (moderate) and 50°C (severe) exerts more profound reduction. Pig bladders were obtained from a local abattoir and stored in cold Tyrode’s solution (95%O2:5%CO2,4°C). The ventral wall was opened longitudinally and bladder wall preparations (10x20mm) were dissected and mounted in a horizontal trough superfused with Tyrode’s (10ml/min). Isometric contractions were recorded at 37°C or during heating, generated by a heating coil placed directly above the superfusate surrounding the strip. Strips were exposed to either repeated 15 minute 50°C exposures or successive 42, 46 and 50°C with 30 minutes at 37°C between interventions. The amplitude, area under the curve (AUC) and frequency (f) of spontaneous contractions were measured for 10 minute periods at the end of control or heating periods. Time controls were done with no heating. Data are mean±S.D., the null hypothesis was rejected at p<0.05 using paired Student’s t-tests. Repeated exposure of severe heating increased the amplitude and AUC (156.1±95.2% and 148.5±110.9% of control, respectively; n=7). There was no recovery during return to 37°C but subsequent heat exposures further increased the amplitude and AUC. By contrast, mild heating to 42°C reduced both the amplitude and AUC to 41.5±20.7% and 43.2±29.2% of that at 37°C; n=3. Recovery to 37°C was incomplete after 30 minutes. Moderate heating to 46°C had no significant effect on either the amplitude or AUC but there was a significant increase during the recovery phase (119.8±15.5% and 118.9±24.4% of control, respectively; n=3). There was no effect on f during heating or upon return to 37°C. These experiments show that the magnitude to which isolated bladder strips are heated have profoundly different effects on the magnitude of spontaneous contractions. Relatively mild heating to 42°C attenuated the amplitude of contractions, an effect that persisted for at least twice the duration of the heating interval itself. However, high heating (46 and 50°C) increased contraction amplitude, an effect that also persisted following the intervention. This implies that heating influences detrusor contractility by several mechanisms that are currently under investigation. Moreover, the design of heating paradigms to regulate spontaneous activity requires precise temperature ranges to be applied to the bladder.