Chronic single-unit recordings were obtained from the mitral cell layer of the olfactory bulbs of awake freely moving rats placed in an odorant stream. Over periods up to five days, 618 recordings from 186 single neurons were obtained. Responses of individual neurons were found to be quite variable over time, although this variability was below chance and was not incremental. The responses of nearby neurons were more similar than expected by chance but less similar than individual neurons recorded at different times. However, responses of spatially well-separated neurons were more different than chance over short time periods. During rapid sniffing, single-unit responses became more variable, and the spatial organization of responses became less apparent. These results suggest that neuronal responses in the olfactory bulb are generally quite variable over time, with this variability increasing during periods of rapid sniffing. These results are interpreted in the context of a distributed, centrally modulated model of olfactory processing.