The unit activity of 16 neurons, including 12 likely mitral cells, was recorded in one olfactory bulb of 5 unrestrained rats, together with contralateral multiunit activity, and respiratory rhythm. The hungry animals were stimulated either by food odor (F) learned as a signal for one available food pellet, or by isoamyl acetate (IA), presented randomly. Eighty-four sequences, each with one stimulation, were analyzed to determine how the odors modified unit, multiunit and respiratory activities. The stimulation could change the variance and/or mean of the unit discharge and its correlation with the respiration phase or frequency or with multiunit activity. Besides the IA sequences, F+, F= and F-situations had to be distinguished, when the pellet was eaten spontaneously, accepted if presented at mouth or refused actively. The neuron responses were reproducible, in a given situation, but their occurrence within a series of identical stimuli was unpredictable from the controlled or observed events. The positive mitral responses were more probable in the F+ and F= than in the F-sequences; they were then associated with multiunit and respiratory activation. Response criteria and neuron typology are discussed. The functional involvements of neuron variability and modulation with internal state are considered.