The development and in vivo test of a fully integrated differential CMOS amplifier, implemented with standard 0.7-microm CMOS technology (one poly, two metals, self aligned twin-well CMOS process) intended to record extracellular neural signals is described. In order to minimize the flicker noise generated by the CMOS circuitry, a chopper technique has been chosen. The fabricated amplifier has a gain of 74 dB, a bandwidth of 3 kHz, an input noise of 6.6 nV/(Hz)0.5, a power dissipation of 1.3 mW, and the active area is 2.7 mm2. An ac coupling has been used to adapt the electrode to the amplifier circuitry for the in vivo testing. Compound muscle action potentials, motor unit action potentials, and compound nerve action potentials have been recorded in acute experiments with rats, in order to validate the amplifier.