The use of HSV1 as a retrograde transneuronal marker for the CNS was assessed in several neuroanatomical systems of the rat brain including the olfactory, visual and somatosensory systems. In all systems, retrograde transneuronal transport was observed; with appropriate survival times transport was evident in third and fourth order neurons in established neuronal circuits. A striking observation was the high frequency of neurons labelled in a Golgi-like manner. The visualization of even the finest dendritic processes provides information about the architecture of neurons several synapses removed from the site of injection. The Golgi-like labelling is so complete that it is possible to identify and process distal parts of dendrites for EM analysis. Thus, it should be feasible to identify synaptic inputs to the dendrites of neurons two or more synapses removed from the site of injection. There was spotty evidence for anterograde transport but the vast majority of the labelling could be accounted for by retrograde transport. With increased survival time, some regions, especially those located one synapse removed from the injection site, became necrotic and the virus spread to glia cells in addition to neurons in those regions. However, in regions more than one synapse removed from the injection there was negligible labelling of glial cells. Taken together, these results suggest that transneuronal retrograde labelling with HSV1 is a useful tool in the analysis of neural circuits.