Mitral/tufted (M/T) cells of the main olfactory bulb transmit odorant information to higher brain structures. The relative timing of action potentials across M/T cells has been proposed to encode this information and to be critical for the activation of downstream neurons. Using ensemble recordings from the mouse olfactory bulb in vivo, we measured how correlations between cells are shaped by stimulus (odor) identity, common respiratory drive, and other cells' activity. The shared respiration cycle is the largest source of correlated firing, but even after accounting for all observable factors a residual positive noise correlation was observed. Noise correlation was maximal on a ∼100-ms timescale and was seen only in cells separated by <200 μm. This correlation is explained primarily by common activity in groups of nearby cells. Thus, M/T-cell correlation principally reflects respiratory modulation and sparse, local network connectivity, with odor identity accounting for a minor component.itral/tufted cells (M/Ts) of the olfactory bulb (OB) receive odor-evoked activity from sensory neurons and transmit it to central brain structures. Thus, understanding how odor information is represented by these neurons' activity is essential to understanding olfactory coding. Studying coding properties at this stage in the olfactory system is particularly interesting because the small number of M/Ts (∼50,000) compared with sensory neurons (∼10 million) or olfactory cortical neurons (∼2 million) suggests that this stage represents a bottleneck (1).Odor information is encoded in the spatial pattern of activity across the OB (2). However, the timing of M/T activity may also play a crucial role in odor representation. Individual M/Ts fire odor-specific patterns of spikes (3), and spike timing across populations of M/Ts relative to the respiration cycle has been proposed as an olfactory code (4, 5). However, whether odor identity influences the correlation of M/T activity (i.e., the tendency of neurons to spike together) has not been specifically addressed.Ensemble firing patterns better predict odorant identity than do single neuron firing rates alone (6, 7), suggesting the utility of a population timing code. Additionally, learned olfactory behaviors are associated with increased M/T spike synchrony (8), and disrupting this synchrony in insect M/T analogs reduces odor discriminability (9). Furthermore, analysis of neural correlations has informed our understanding of the relationship between neural circuits and population activity and has constrained hypotheses concerning "decoding" of incoming population activity by downstream areas (10).Here, we evaluated how relative M/T timing depends upon odor identity and timing, respiration phase (inhalation/exhalation), and other neurons' spiking. Correlated spiking in the OB is familiar (11, 12), but how these correlations depend on such variables is unknown. Correlations may originate in common stimulus or respiration phase preferences ("signal correlation"). Cell pairs' spiking may al...