The integration of implantable CMOS neural probes with thousands of simultaneously recording microelectrodes is a promising approach for neuroscience and might allow to literally image electrophysiological neuronal activity in multiple brain circuits as we have previously shown in vitro. Here, we present a complete system based on a fully multiplexed CMOS neural probe that was designed for in-vivo acute recordings with a scalable circuit architecture. In particular, a first prototype of a single-shaft probe with 512 electrodes was realized in a standard CMOS 0.18μm technology and post-processed to structure the shaft with a wedge-like geometry of 30μm in thickness at the tip and 80μm at the base. The design of the system and of the probe as well as the post-processing techniques are discussed. Finally, preliminary results on electrical, mechanical and implantation tests are presented to demonstrate the feasibility of our approach.