Olfactory learning is associated with substantial neural changes at the level of the accessory and main olfactory bulb, during both pheromonal learning in mated mice and lamb odour recognition in post partum sheep. These forms of learning occur during "sensitive periods" and an important question is whether similar neural changes occur in the olfactory bulb at other times. We used a classical conditioning procedure to establish an olfactory discrimination in adult mice and then measured changes in neurotransmitter levels in the main olfactory bulb in response to the presentation of the conditioned odours. Presentation of the conditioned, but not the non-conditioned, odour resulted in significant increases in the levels of certain transmitters, including glutamate from the mitral/tufted cells, GABA from the granule and periglomerular cells and noradrenaline from the centrifugal projection from the locus coeruleus. Overall, there was a decrease in the ratio of excitatory to inhibitory neurotransmitters in the olfactory bulb in response to the conditioned, but not the non-conditioned odour. Moreover, the magnitude of the decrease in this ratio was correlated with the level of behavioural response to the conditioned odour. These findings support the hypothesis that changes in the gain of the reciprocal synapses between mitral/tufted neurons and their inhibitory interneurons are a general feature of olfactory learning.