PROJECT SUMMARY. Transneuronal transport of neurotropic viruses is uniquely able to reveal the cellular composition of multi-synaptic circuits. As a consequence, this technique is providing fundamental new insights into the functional architecture of sensory, motor, cognitive and affective networks in the central nervous system. The Center for Neuroanatomy with Neurotropic Viruses (CNNV) was created in 2004 to provide the neuroscience community with access to the highly specialized reagents, training and facilities that are necessary to use neurotropic viruses as transneuronal tracers. We document in this application that the CNNV has become an essential national and international resource for an important segment of the neuroscience community. Users of the Center are funded by 15 different institutes of the NIH, by NSF and by private foundations as well as international research organizations. The Center has established a strong history of providing: 1) well-characterized reagents, 2) training and guidance in the use of the method, and 3) specialized biohazard facilities for performing experiments using neurotropic viruses. The Center also has played a vital role in the development of new and improved transneuronal tracing tools that permit more targeted dissection of the functional architecture of neural circuits. Furthermore, the Center has served as a critical technical and intellectual resource for those interested in using this method. The goal of this competitive renewal is to enable the Center to continue both its resource and research functions.