1. Response properties of rat olfactory bulb neurones were studied by recording single-unit activity evoked by stimuli delivered by an air-dilution olfactometer. Neural responses were quantified as peristimulus time histograms by averaging the responses evoked by eight presentations of the same stimulus. Animals were stimulated by means of controlled artificial sniffs produced by a vacuum source connected to the choana.2. Evoked activity was divided into on- and after-responses. Three types of on-responses were distinguished on the basis of temporal patterns of activity evoked by odorous stimuli. Neurones exhibited the same type of response when driven by different odorants. For a given neurone, the size of an evoked response depended on the odorant used as a stimulus substance. The stimulating effectiveness of particular odorants varied between different neurones.3. After-responses were divided into two classes: phasic (0.5-3 sec after stimulus offset) and tonic (3 sec-2 min after stimulus offset). After-responses were associated with a subset of neurones making each type of on-response and with units producing no clear on-response. It is suggested that the occurrence of an after-response in a given neurone is not dependent on the nature of the evoked on-response.4. Intensity-related changes of on-responses were characterized by three measures: number of action potentials evoked during the stimulus event, latency of action-potential bursts, and rate of activity during the action-potential bursts. The number of evoked action potentials was not monotonically related to stimulus concentration. In contrast, there was a clear relationship between the pattern of evoked activity and stimulus concentration.5. It is proposed that the timing of excitation and inhibition is dependent on the concentration of the stimulus and that the number of evoked action potentials depends on the odorant used as a stimulus substance and the concentration at which it is delivered.