Single unit activities from the olfactory bulb were recorded during sniffing in unanesthetized rats. Spectral analyses by the Walsh transform were applied to the spike trains of units. In addition, inter-spike intervals (ISIs) were measured to estimate post-spike excitability and serial correlations. The modulation of firing due to respiration cycles was observed only in four units (10%). Thirty-three units (82.5%) displayed low-frequency fluctuations such as 1/f noise in spike density. Their spectral density for the low-frequency range was inversely proportional to the frequency, to the power of 0.08-0.55. These units also showed "hyper-recovery" in post-spike excitability. In addition, the first-order serial correlation of ISIs was significantly positive in 36 units (90%). The degree of 1/f fluctuation was significantly correlated with the amplitude of "hyper-recovery" in post-spike excitability and with the serial correlation coefficient. To test the hypothesis that the self-facilitation of a neuron causes 1/f fluctuations in its spike trains, simulations of single neuron activity with self-facilitation were performed using an integrate-and-fire neuron model, and its spike trains were analyzed. The model neuron with self-facilitation mimicked the characteristics observed in olfactory bulb neurons such as 1/f fluctuations in spike density, positive correlations between successive ISIs and "hyper-recovery" in post-spike excitability. These results suggest that olfactory bulb neurons have a mechanism for self-facilitation causing 1/f fluctuations in spike density.