One of the first steps in the coding of olfactory information is the transformation of synaptic input to action potential firing in mitral and tufted (M/T) cells of the mammalian olfactory bulb. However, little is known regarding the synaptic mechanisms underlying this process in vivo. In this study, we examined odor-evoked response patterns of M/T and granule cells using whole-cell recording in anesthetized, freely breathing rats. We find that odor-evoked excitatory responses in M/T cells typically consist of bursts of action potentials coupled to the approximately 2 Hz respiration rhythm. Odor-evoked, rhythmic M/T cell excitation is reliable during odor presentation (2-4 sec); in contrast, both excitatory responses of granule cells and M/T cell lateral inhibition adapt quickly after the first respiration cycle in the presence of odorants. We also find that the amplitude and initial slope of odor-evoked synaptic excitation play an important role in regulating the timing of M/T cell spikes. Furthermore, differences in odor concentration alter the shape of odor-evoked excitatory synaptic responses, the latency of M/T cell spikes, and the timing of M/T cell lateral inhibition.