Odors synchronize the activity of olfactory bulb mitral cells that project to the same glomerulus. In vitro, a slow rhythmic excitation intrinsic to the glomerular network persists, even in the absence of afferent input. We show here that a subpopulation of juxtaglomerular cells, external tufted (ET) cells, may trigger this rhythmic activity. We used paired whole-cell recording and Ca(2+) imaging in bulb slices from wild-type and transgenic mice expressing the fluorescent Ca(2+) indicator protein GCaMP-2. Slow, periodic population bursts in mitral cells were synchronized with spontaneous discharges in ET cells. Moreover, activation of a single ET cell was sufficient to evoke population bursts in mitral cells within the same glomerulus. Stimulation of the olfactory nerve induced similar population bursts and activated ET cells at a lower threshold than mitral cells, suggesting that ET cells mediate feedforward excitation of mitral cells. We propose that ET cells act as essential drivers of glomerular output to the olfactory cortex.