The transduction mechanism of the conductance activated by saccharin was analysed in isolated bullfrog taste cells under whole-cell voltage-clamp. Bath application of 30 mm saccharin induced an inward current of −34 ± 12 pA (mean ± SEM, n = 10) at a membrane potential of −50 mV in 10 (23%) of 44 rod cells. The concentration–response relationship for the saccharin-gated current was consistent with that of the gustatory neural response. The saccharin-induced current was accompanied with a conductance increase under internal low Cl– condition (ECl = −56 mV), suggesting that saccharin activated a cation conductance. The reversal potential of the saccharin-induced current was −17 ± 2 mV (n = 10). Intracellular dialysis of 0.5 mm guanosine 5′-O-(2-thiodiphosphate) (GDP-β-S) completely blocked the saccharin-induced response, suggesting the involvement of a G protein in the transduction. The dialysis of heparin (1 mg/mL) also inhibited the response almost completely, but the dialysis of 1 mm 8-Br-cAMP did not affect the response significantly. Intracellular 50 µm inositol 1,4,5-trisphosphate (1,4,5 InsP3) also induced the inward current in five (38%) of 13 rod cells, but intracellular Pasteurella multocida toxin (5 µg/mL, Gαq-coupled PLC activator) did not elicit any response in the cells. The results suggest that saccharin mainly activates a cation conductance in frog taste cells through the mediation of IP3 production.