This study investigates in detail how mitral cell activity is distributed during the respiratory cycle in freely breathing animals and how this temporal pattern changes under odour stimulation. Results were obtained from 1408 sequences composed of a 30-s period of spontaneous activity followed by a 10-s period of stimulation. Spontaneously, a majority of the patterns did not show any clear relationship with respiration and were categorized as unsynchronized. Stimulations evoked a high proportion of synchronized patterns. About 40% displayed a single period of firing rate increase superimposed on no background activity or on sustained background activity, and were categorized as simple excitatory synchronized patterns. Thirty-six per cent showed a single inhibitory trough, and were categorized as simple suppressive synchronized patterns, whereas the remaining 24% showed a succession of peaks and troughs, and were categorized as complex synchronized patterns. Under pure air delivery, the position in time of the firing peak in simple excitatory synchronized patterns was found to be generally phase-locked on late inhalation and early exhalation. During stimulation, its position did not change in patterns which originated from spontaneous patterns having the same type whereas it was shifted towards earlier portions of the cycle in patterns originating from another type. Lastly, the possibilities of transition between spontaneous and odour-evoked patterns seemed to follow general rules. Whereas any type of spontaneous patterns may transform into any other type under stimulation, a majority of synchronized odour response patterns originated from unsynchronized spontaneous patterns. This may reflect some potential of cells having a non-modulated spontaneous activity to be more responsive to peripheral inputs.