Macromolecule-conducting pores have been recently recognized as a distinct class of ion channels. The poor role of macromolecules as electrical charge carriers can be used to detect their movement along electrolyte-filled pores. Because of their negligible contribution to electrical ion currents, translocating macromolecules reduce the net conductivity of the medium inside the pore, thus decreasing the measured pore ion conductance. In the extreme case, a large translocating macromolecule can interrupt ion flow along the pore lumen, reflected as a negligible pore conductance. Therefore, ion conductance serves as a measurement of macromolecular transport, with lesser values indicating greater macromolecular translocation (in size and/or number). Such is the principle of operation of the widely used Coulter counter, an instrument for counting and sizing particles. It has long been known that macromolecules translocate across the central channel of nuclear pore complexes (NPCs). Recently, large conductance ion channel activity (100-1000 pS) was recorded from the nuclear envelope (NE) of various preparations and it was suggested that NPCs may be the source of this activity. Despite its significance to understanding the regulation of transcription, replication, mRNA export, and thus gene expression of normal and pathological states, no report has appeared demonstrating that this channel activity corresponds to ion flow along the central channel of the NPC. Here we present such a demonstration in adult mouse cardiac myocyte nuclei. In agreement with concepts introduced for macromolecule-conducting channels, our patch clamp experiments showed that ion conductance is reduced, and thus that ion flow is restricted during translocation of macromolecules containing nuclear targeting signals. Ion flow was blocked by mAb414, a monoclonal antibody raised against a major NPC glycoprotein and known to localize on the NPC channel where it blocks macromolecular transport. These results also establish patch clamp as a useful technique for the measurement of macromolecular translocation along the large central channel of the NPC and provide a basis for the design of future investigations of nuclear signaling for control of gene activity, mRNA export for gene expression, as well as other processes subservient to NPC-mediated nucleocytoplasmic exchange.