Although high-resolution tomographs provide a new approach that strongly simplifies the measurement of in vivo tracer biodistribution and kinetics in small animals, they suffer from an important drawback: the need for animal anesthesia or immobilization, which restricts the neurophysiological investigations. Furthermore, quantitative irt vice experiments realized on the brain sometimes only require a simple measurement of the radioactivity achieved on a few local points and do not necessarily imply the use of a tomograph, which is a detector of high cost, These constraints led us to develop an intracerebral beta sensitive probe, sonde intracerebrale (SIC) (French acronym of intracerebral probe) that will allow chronic measurements of the neurophysiological activity in awake and unrestrained small animals. The volume to which the probe is sensitive and the noise contributions to the relevant signal have been evaluated through Monte Carlo simulations. Characterizations of a first prototype based on a small piece of scintillating fiber (500-mu m diameter and 1-mm length) fused to a same diameter optical fiber coupled in turn to a photomultiplier are also presented. A first configuration of the detector is finally proposed.