The olfactory system influences a wide range of fundamental behaviors including food finding, reproduction, fear response and social communication. These are regulated by the mixtures of odorant molecules which stimulate olfactory receptor neurons (ORN) and axons of these neurons project into the glomerular layer of the main olfactory bulb. Within the glomeruli, axons of ORN synapse onto the apical dendrites of mitral and tufted cells in the main olfactory bulb. The information is then passed on to the primary olfactory cortex including the anterior olfactory nucleus (AON), the piriform cortex, amygdala and the entorhinal cortex. However, how the information is transduced and processed in each brain region is not well known. Previously, we reported that the rat AON contains many interneurons which express vasopressin, and that these show an increase in the expression of the immediate early gene product, EGR-1, when exposed to social, but not predator odour, suggesting that these vasopressin neurons may be selectively involved in the coding of social odour information (1). To better understand the processing of information in the AON, we recorded spontaneous firing activity from single AON neurons by in vivo extracellular electrophysiology. All experiments were performed on adult male Sprague-Dawley rats (~ 450 g) under urethane anaesthesia (ethyl carbamate, 1.3 g/kg i.p.). We studied their discharge patterning by constructing hazard functions, which show how the excitability changes with time after a spike. Almost all AON cells fired in repeated clusters of spikes separated by very short intervals (< 33 ms), and this was reflected in hazard functions with a pronounced early peak. These cells could be subdivided into 3 classes: in one class, the hazard function was uniformly flat after the early peak, reflecting random occurrence of clusters; another class showed a second peak hazard at ~600 ms reflecting clusters occurring at regular intervals with a frequency of ~ 2 Hz, a third class showed peaks at both 600 ms and 1200 ms, indicating an oscillatory drive to these cells at ~ 2 Hz. The origin of this rhythm is unclear, as it is substantially faster than the respiratory rhythm that drives mitral cells in the olfactory bulb. The firing rate for the 3 classes are 4.3 ± 0.5 Hz (n = 71), 9.1 ± 0.7 Hz (n = 69), 2.6 ± 0.2 Hz (n = 51) respectively. We are now testing how these 3 distinct subpopulations differ in their processing of social odour information.