Recordings of single unit activity before, during and after a 1-h continuous exposure were made from the mitral cell layer of the olfactory bulbs of awake rabbits. Thirteen complete 1-h exposures were performed using 8 neurons, while a further 6 neurons were lost before the end of the exposure. During the first few minutes of exposure, a respiration-related synchronization of firing activity occurred which was similar to that of the pre-exposure 5-s stimulations with the same odor. This discharge patterning continued for the full duration of some exposures but disappeared within a few minutes for others. In all cases, a response to the exposure odor, although reduced in amplitude, was evident 6 min after the exposure had been terminated. This reduction was also observed for stimulations with other odors that had induced a response prior to exposure, and was always followed by a gradual recovery in the amplitude of the response. The relation between the changes in the firing activity of mitral cells during and after 1-h exposures, and adaptation phenomena observed using psychophysical methods are discussed. The absence of gross changes in firing activity are discussed with reference to 'mitral cell selective degeneration' and autoradiographic 2-deoxyglucose experiments.