Nitric oxide (NO) is a potent central neuromodulator of respiration, yet its scope and site of action, are unclear. We used 7-nitroindazol (7-NI), a selective inhibitor of endogenous neuronal NO synthesis, to investigate the neurogenesis of respiration in larval bullfrog (Rana catesbeiana) isolated brain stems. 7-NI treatment (0.0625 to 0.75 mM) increased the specific-frequency of buccal ventilation (BV) events, indicating influence on BV central rhythm generators (CRG). The drug reduced occurrence, altered burst shape and disrupted clustering of lung ventilation (LV) events, without altering their specific-frequency. LV bursts occurrence and clustering also differed between pH conditions. We conclude that NO has diverse effects on respiratory rhythmogenesis, being necessary for the expression of respiratory rhythms, inhibiting the frequency of BV CRG, and affecting both shape and clustering of LV bursts through conditional modulation of LV CRG. We confirm central chemosensitivity in these preparations, and demonstrate chemo-modulation of LV burst clustering and occurrence, but not specific-frequency. Results support distinct oscillators underlying LV and BV CRGs.