Inward currents were studied in interstitial cells isolated from bladders of humanely killed guinea-pigs. These cells have previously been shown to be kit-positive (McCloskey & Gurney, 2002) and several components of outward potassium current have recently been characterised (McCloskey, 2005). Enzymatic dispersal of detrusor tissue produced a heterogeneous yield of cells containing smooth muscle cells and interstitial cells. The latter were readily identified by the presence of several lateral branches and were studied using the whole-cell patch clamp technique and Cs⁺-filled pipettes. Data are expressed as means ±S.E.M. and were compared with Student’s paired t test. P values less than 0.05 were considered significant. Inward currents were evoked by stepping positively from a holding potential of −80mV. They activated at −50mV, peaked at 0mV and reversed positive to 50mV and were half-maximally activated at −27±1mV (n=15). The inward current displayed voltage-dependence of inactivation with half-maximal inactivation at −36mV. Boltzmann fits of the activation and inactivation data revealed a window current between −40mV and 20mV. The time course of decay of the current evoked at 0mV could be fitted with a single exponential and had a time constant of 79±11ms (n=4). Replacement of external Ca²⁺with Ba²⁺significantly increased this to 344 ±61ms (n=4, p=0.022). The current amplitude was also augmented by Ba²⁺ (n=6). Removal of extracellular calcium significantly reduced inward currents (n=5). In contrast, they were enhanced by Bay K 8644 (1μM, n=3). Nifedipine (1μM), significantly reduced current amplitude across the voltage range; however, blockade was more effective on the current evoked at 0mV (−58±7pA to −19±4pA, n=4, p=0.004) than that evoked by a step to −20mV (−39±3pA to −25 ±2pA, n=4, p=0.012) either indicating voltage dependence of the action of nifedipine or another component of inward current. Increasing the concentration of the drug to 10μM did not bring about further significant reduction either at 0mV or at ±20mV. In the presence of 1μM nifedipine, however, the latter current, was significantly reduced by 100μM Ni²⁺ (− 18±4pA to -5±3pA, n=10, p=0.008). In summary, interstitial cells from the guinea-pig detrusor possess inward currents with typical characteristics of L-type Ca²⁺ current. They also had a component of inward calcium current which was resistant to nifedipine, but sensitive to Ni²⁺. Further work is required to characterise the latter conductance.