Interactions between brainstem respiratory neurons underlie respiratory rhythmogenesis. Several techniques to study these interactions are discussed. Cross-correlation analysis of the firing patterns of pairs of respiratory neurons revealed a high probability of interaction and/or shared inputs between neurons that are immediate neighbors, and the lack and ubiquitous connections between neurons located in different regions. Neuroanatomical studies demonstrated a previously unobserved brainstem projection to the ventrolateral nucleus of tractus solitarius, an important brainstem respiratory-related structure, from a small cluster of neurons in the contralateral ventrolateral medulla several millimeters rostral to the obex; subsequent recordings have shown these neurons to have expiratory firing patterns. Modeling studies suggested that neurons hypothesized to produce inspiratory phase termination should begin firing late in the inspiratory phase and should be excited by lung inflation. Subsequent extracellular recordings have shown that neurons with these properties constitute a significant fraction of neurons in the region of te ventrolateral nucleus of tractus solitarius. The results of these studies indicate that rhythmogenesis results from highly specific interactions between limited subpopulations of respiratory neurons.