Iontophoresis of hydrogen ions （H￣+ ） to respiration related units （RRUs） in the nucleus retrofacialis （NRF）region was used to investigate the role of RRUs in central chemoreception of respiration. The results were as follows:1) of the 1 3 inspiration related units （InRUs） recorded

7 were excitatory and 1 was inhibitory; 2) of the 29 expiration related units （ExRUs）. 12 were inhibitory and 1 was excitatory; and 3) of the 7 phase-spanning respiration related units （PSUs）. 4 were excitatory and 2 were inhibitory. None of the remainder were affected.Histochemical study using the horseradish peroxidase （HRP） technique revealed a neural structural relation between the ventral surface of the medulla and neurons in the NRF region. It was suggested that the RRUs in the NRF region might contribute to the central chemoreception which regulates respiration.