In a previous study, we showed that in an in vitro en bloc preparation of newborn rats perfused with standard [K(+)] (6.2mM) and high [K(+)] (11.2mM) artificial cerebrospinal fluid (aCSF), nociceptin/orphanin FQ (N/OFQ) suppresses bursting of pre-inspiratory neurons with 1:1 coupling to the fictive inspiration. However, it is unclear whether the pre-Bötzinger complex (preBötC) is involved in the N/OFQ-induced slowing. Using in vitro en bloc preparations with and without the retrotrapezoid nucleus/parafacial respiratory group (RTN/pFRG) perfused with high [K(+)] aCSF, we found the following: (1) there were no differences in the effects of N/OFQ on the inspiratory rhythm between the preparations with and without the RTN/pFRG, (2) N/OFQ decreased the input resistance of inspiratory neurons (Insps) in the preparations without the RTN/pFRG and suppressed their ectopic firing activities, and (3) N/OFQ suppressed the spontaneous firing of Insps under a chemical synaptic transmission blockade. In conclusion, it is possible that the preBötC is involved in N/OFQ-induced inspiratory rhythm slowing.