The changes in single unit activity induced by short odour presentations were studied in the olfactory bulbs of awake rabbits. 1. It was found that an accurate evaluation of the cell responses required the separate processing of the inspiration- and expiration-related activity. 2. Three main types of response were observed during odour presentation. One was characterized by an increase in firing over several successive inspirations, with a decrease in the corresponding expiratory phases. The other two were characterized by an inspiration-related increase or decrease in activity without any modifications during expiration. 3. The end of odour presentation led to inconsistent "off-effects" in the cells affected by the stimulation. The most conspicuous were: a complete cessation of the firing for several respiratory cycles following an excitation, an excitatory rebound following inhibition, a reversal of the respiratory phase-related discharge, i.e., activity decrease during inspiration and the increase during expiration. 4. The findings are discussed with reference to previous studies on anaesthetized animals, together with their relevance to sensory coding in the olfactory bulb.