1. Single-unit activity was recorded from olfactory bulb neurones driven by odorous stimuli. Neural responses were quantified as averaged peristimulus time histograms. 2. Successive presentations of the same stimulus evoked similar patterns of activity during each presentation. Some neurones exhibited increased and others decreased excitability in the adapted state. The occurrence of such facilitative or suppressive self-adaptation was not determined by odorant concentration or by the number of action potentials evoked by a given stimulus. Neurones driven by more than one odorant exhibited the same type of adaptation (facilitative or suppressive) for all effective stimuli. 3. When the first odorant differed from the second, the number but not the pattern of action potentials evoked by the second odorant differed from the non-adapted state. Some neurones exhibited increased and others decreased excitability in the cross-adaptation state. Neurones exhibiting suppressive self-adaptation did not exhibit facilitative cross-adaptation and those exhibiting facilitative self-adaptation did not exhibit suppressive cross-adaptation. Instances of asymmetrical cross-adaptation were noted in which two odorants differed in the extent to which they affected subsequent sensitivity to each other. The degree of symmetry for a given pair of odorants differed for different neurones. It is concluded that neurones in the olfactory bulb differ in both the type (suppressive or facilitative) and extent of adaptation evoked by a given odorant.