We present a CMOS-based microelectrode array with 11'016 metal electrodes and 126 on-chip channels, each of which includes recording and stimulation electronics for bidirectional communication with electrogenic cells (neurons or cardiomyocytes). The features of this chip include high spatial resolution with 3200 electrodes per mm" to attain cellular or subcellular resolution (electrode diameter 7 mum, pitch 18 mum, honeycomb pattern), great flexibility in routing the 126 channels to the 11'016 recording sites, and low noise levels in the recordings (2.4 muVnns) so that single action potentials from mammalian cells can be monitored. The low noise levels also enable the recording of single-unit spike activity in acute slice preparations.