Our previous studies showed: electrograms traces made with wire (WE) and catheter electrodes (CE) at sites where the atrial and specialized tissues overlap contain waveforms evoked by both tissues. At the atrionodal bundle (AB) site, the so-called low-high amplitude (amp) potentials are seen. The low waveforms reflect AB activation, which persists with high K, the high-amp potentials reflect atrial activation and ceased with high K. Transmembrane potentials (TAPs) made deep to WE at sites where coincident CE and WE traces are found, showed AB TAPs are plateau-shaped and atrial potentials (Ap) are triangular. Using coincident CE, WE, and ME recordings for targeting ABs for Lucifer Yellow (LY) iontophoresis/photoablations, we found some plateau potentials were coincident with the high-amp Ap. Here we report the electrical and anatomic features of myocytes revealed by LY. Methods and Results Standard methods were used for CE and WE recordings in the unipolar mode and for TAPs. Myocytes targeted by their TAPs were LY injected for 10 mins using 20 to 40 nA hyperpolarizing current. Goldner trichrome and unstained serial transverse sections were viewed by light and epifluorescent microscope. LY was confined to (a) sub-endocardial atrial fascicles when coincident high-amp WE, CE and triangular TAPs were targeted; (b) sub-epicardial atrial fascicles when high-amp and plateau TAPs were targeted; and (c) AB fascicles when coincident low-amp and plateau potentials were targeted. The AB potentials displayed a more prominent spike phase 1 and dome than the atrial plateaus. Conclusions Some ordinary atrial myocytes possess plateau potentials. Coincident CE, WE, and ME recordings are essential for delineating the atrial from the AB myocytes for in-vitro studies. Support: NIH HL073261 (DKR).