In the ongoing study we are applying a modified method of intracerebral microinjection that permits improved spatial resolution. This allows us to better localize the anesthetic-sensitive locus to a small dorso-caudal sub-region of the MPTA. When applied there, the GABAergic agents muscimol, pentobarbital and propofol were all effective. With accurate targeting it is sufficient to expose as few as 1700 MPTA neurons to the drug in order to sustain (whole-body) general anesthesia. The effective locus is centered on a tegmental (reticular) field, traversed by fibers of the superior cerebellar peduncle, which has no specific nuclear designation and has not previously been implicated in brain state transitions. Characterizing the intrinsic properties and connectivity of this population of MPTA neurons might lead to better understanding of the circuitry involved in transitioning between the awake and the unconscious states.