The activity of 219 cells in the olfactory bulb of waking, freely breathing mice was analysed, and it was found that their spontaneous discharge rate varied between 0.3 and 33 AP/s. Both butyric acid and geraniol elicited responses of four types: 38% of the 98 responses were of type 1 (excitation), 43% of type 2 (inhibition), 10% of type 3 (complex structure), and 8% were characterized by a change in the temporal pattern of the activity. Response duration varied from less than 500 ms to more than 1 min. 52 secondary neurons were stimulated with four different concentrations of the odor substances. All of the responsive cells showed a clear ability to discriminate concentration. That is, response magnitude varied with intensity, producing non-monotonic curves. Most of the neurons responded only in a region of a more or less limited concentration, and in no case was a saturation curve observed. Approx. 40% of the neurons responded to the lowest concentration tested (10(-8) vol.% of butyric acid or geraniol). The strongest stimuli, 10(-2) vol.%, were relatively ineffective.