Inositol 1,4,5-trisphosphate (InsP(3)) receptors (InsP(3)Rs) are intracellular Ca(2+) channels gated by the second messenger InsP(3). Here we describe a novel approach for recording single-channel currents through recombinant InsP(3)Rs in mammalian cells that applies patch-clamp electrophysiology to nuclei isolated from COS-7 cells transiently transfected with the neuronal (SII(+)) and peripheral (SII(-)) alternatively-spliced variants of the rat type 1 InsP(3)R. Single channels that were activated by InsP(3) and inhibited by heparin were observed in 45% of patches from nuclei prepared from transfected cells overexpressing recombinant InsP(3)Rs. In contrast, nuclei from cells transfected with the vector alone had InsP(3)-dependent channel activity in only 1.5% of patches. With K(+) (140 mM) as the permeant ion, recombinant SII(+) and SII(-) channels had slope conductances of 370 pS and 390 pS, respectively. The recombinant channels were 4-fold more selective for Ca(2+) over K(+), and their open probabilities were biphasically regulated by cytoplasmic [Ca(2+)]. This approach provides a powerful new methodology to study the permeation and gating properties of recombinant mammalian InsP(3)Rs in a native mammalian membrane environment at the single-channel level.