The aim of the present study was to determine the characteristics of the fast oscillations in the juvenile rat phrenic nerve, and to establish their temperature- and state-dependence. Two different age-matched decerebrate, baro- and chemo-denervated rat preparations, the in vivo and the in situ arterially-perfused models, were used to examine three systemic properties: generation and dynamics of fast oscillations in phrenic (Ph) activity (both preparations); responses to anoxia (both preparations); and the effects of temperature on fast oscillations (in situ only). Both juvenile preparations generated power and coherence in two major bands analogous to adult high- (HFO) and medium-frequency oscillations at frequencies that increased with temperature but were lower than in adults. At < 28 °C, however, Ph oscillations were confined primarily to one low-frequency band (20-45 Hz). During sustained anoxia, both preparations produced stereotypical state changes from eupnea to hyperpnea to transition bursting (a behavior present only in vivo during incomplete ischemia) to gasping. Thus, the juvenile rat produces a sequential pattern of responses to anoxia that are intermediate forms between those produced by neonates and those produced by adults. Time-frequency analysis determined that fast oscillations demonstrated dynamics over the course of the inspiratory burst, and a state-dependence similar to that of adults in vivo, in which hyperpnea (and transition) bursts are associated with increases in HFO, while gasping contains no HFO. Our results confirm that both the fast oscillations in Ph activity and the coherence between Ph pairs produced by the juvenile rat are profoundly state- and temperature-dependent.