The olfactory system can perform remarkable tasks in the identification and detection of odors. The circuits that perform these tasks are poorly understood. Recent work has demonstrated important differences in in vivo odor-evoked responses between two types of projection neurons in the olfactory bulb, the mitral and tufted cells. Evidence suggests that differences in the connectivity and local circuit properties of mitral and tufted cells likely play a key role in generating differential stimulus selectivity and response patterning that may be key for olfactory system function. Here we analyze circuit-level differences between olfactory bulb projection neuron types using electrophysiological and optogenetic approaches. We specifically propose the central hypothesis that tufted and mitral cell populations associated with different glomeruli represent parallel, weakly interacting and functionally complementary pathways of olfactory bulb output. We describe experiments and analysis to test specific elements of this hypothesis by examining the local circuit mechanisms that underlie the differences in odor- evoked responses between tufted and mitral cells. We are especially interested in differences related to the inhibitory circuitry of the olfactory bulb. We will test this central hypothesis by performing experiments designed to assess the strength, sources and plasticity of inhibitory connections made onto mitral and tufted cells. In Aim 1 we will determine how lateral inhibition differentially affects spike timing in mitral and tufted cells. We predict that tufted cells will be less strongly inhibited by activation of neighboring glomeruli than will mitral cells. In Aim 2 we will determine the different sources of mitral and tufted cell input. A critical question for understanding olfactory bulb circuitry is whether inhibition onto mitral and tufted cells arises from the same or different cell types. In Aim 3 we will examine differences in experience-dependent plasticity of synapses to mitral and tufted cells. New data that we have collected represents the first example of plasticity in these local circuits. We propose to examine the mechanisms and loci of this plasticity.