The aim of the present study was to assess the size of the input from TRH immunoreactive varicosities to medullary respiratory neurons in the Botzinger complex and caudal ventral respiratory group. Neurobiotin was intracellularly injected into seven neurons in the Botzinger complex, between 0.4 and 0.9 mm caudal to the facial nucleus. Five of the seven Botzinger neurons had extensive local axonal projections, with bouton-like varicosities clustered predominantly between their somata and the nucleus ambiguus. Seven neurons in the caudal ventral respiratory group, located between 1.6 and 2.4 mm caudal to the facial nucleus, were also labelled. All butone caudal respiratory neurons had no, or very few, medullary collaterals. TRH immunoreactive fibres were seen in many medullary nuclei, including the ventral reticular formation. Botzinger neurons were closely apposed by an average of 29 ± 8 TRH immunoreactive boutons/neuron (mean ± S.D., n = 7). In contrast, caudal ventral respiratory group neurons were apposed by only 5 ± 3 TRH immunoreactive boutons/neuron (n = 7). Botzinger neurons form many intramedullary and bulbospinal inhibitory connections with premotoneurons and motoneurons that are important in the timing, amplitude and shape, of respiratory activity. Our findings suggest a role for endogenous TRH-containing neurons in modulating the activity of inhibitory Botzinger neurons and neurons in the caudal ventral respiratory group. The significance of the apparent difference in size of this input remains to be determined.