Insulin and leptin, released into the systemic circulation, are known to have important effects on food intake. Most attention has been given to the hypothalamus as a likely target for those actions but there is evidence that receptors for both leptin and insulin are also densely expressed in the olfactory bulb, indicating that these hormones might affect the processing of sensory information linked to food palatability and pursue. The aim of this study was to assess the action of insulin on mitral cell activity using in vivo extracellular electrophysiology. Adult male Sprague Dawley rats were anaesthetised with urethane (7.5ml/ kg, 10% v/v) and positioned in prone position in a stereotaxic frame. Two holes were drilled, one above the main olfactory bulb (MOB) and one above the lateral olfactory tract (LOT), which contains the axons of the mitral cells, the main output neurons of the MOB. Mitral cells were antidromically identified by stimulation of the LOT and confirmed by their characteristic firing pattern. Above the hole of the recording electrode a tubing containing insulin (30mU, 3μl, 10mU/μl) was positioned, connected to a syringe for topical administration of insulin. Mitral cells fire phasically with intermittent bursts of action potentials which last >100ms which are separated by intervals of variable length between cells during which there is either decreased activity or complete silence. Moreover, they can be divided into two populations. In the first population, within each burst the instantaneous frequency ranges from 80-100Hz. In the second population, within bursts successive spikes of very short interspike intervals (3-8ms) occur resulting in a bimodal interspike interval distribution and a ‘double-band’ instantaneous frequency distribution with the first band being 70-100Hz and the second one being 150-250Hz. We call these short-interval spikes ‘doublets’ and ‘triplets’. Out of 17 cells tested, 12 were not affected by insulin but five showed a marked excitatory response, showing an approximately 8-fold increase in the doublets and triplets. Two cells which did not show high frequency events, started firing doublets and triplets as a result of the insulin’s application. The increase in firing rate was observed about 10 min after application and lasted approximately 10 min before the firing returned to baseline. This study suggests that insulin applied to the olfactory bulb can affect the firing rate of a proportion of mitral cells. We are now testing whether it is a direct effect on mitral cells and whether systemically applied insulin has the same effects. Our results indicate the ability of the olfactory bulb to respond to changes in insulin blood concentrations driven by physiological fluxes that would typically follow a meal, during obese state or metabolic disease.