Synthetic funnel web spider toxin (sFTX-3.3) is a polyamine amide analogue of FTX, a toxin fraction isolated from the venom of the funnel web spider, Agelenopsis aperta, that blocks P-type Ca2+ channels. The structures of these polyamine containing compounds are not identical: sFTX-3.3 contains an amide carbonyl oxygen that is absent from the predicted structure of native FTX. Recently, a compound called FTX-3.3 was synthesized with the structure predicted for native FTX. We have compared the effects of polyamine amide sFTX-3.3 and polyamine FTX-3.3, on Ca2+ channel currents in the soma of mature rat cerebellar Purkinje neurons, in which the predominant Ca2+ channels are defined as P-type. Differential inhibition by sFTX-3.3 and FTX-3.3 revealed three populations of Ca2+ channels. One group, mediating approximately 66% of the current, was blocked by sFTX-3.3 with an IC50 (concentration producing half maximal inhibition) of 33 nM or by FTX-3.3 with an IC50 of 55 pM. A second population (5-25% of the total current) was inhibited by sFTX-3.3 with an IC50 of 33 nM, but was insensitive to FTX-3.3, while a third (10-30%) was blocked by FTX-3.3 with an IC50 of 125 nM and was resistant to sFTX-3.3. These channels also showed distinctive current-voltage relationships. Our results suggest that P-type Ca2+ channels in mature rat cerebellar Purkinje cells may be subdivided according to pharmacological and biophysical properties.