1. Intermittent hypoxia elicits long-term facilitation (LTF) of phrenic motor output in anaesthetized rats. We tested the hypothesis that an equal cumulative duration of continuous hypoxia would not elicit phrenic LTF. 2. Integrated phrenic nerve activity was recorded in urethane-anaesthetized, vagotomized, paralysed and ventilated rats exposed to: (1) 3 X 3 min hypoxic episodes (inspired O2 fraction (FI, O2) = 0.11) separated by 5 min hyperoxia (FI,O2 = 0.5; n = 6), (2) 9 min continuous hypoxia (n = 6), or (3) 20 min continuous hypoxia (n = 7). Isocapnia was maintained throughout the protocol. 3. Consistent with previous studies, phrenic amplitude was significantly elevated for at least 1 h following intermittent hypoxia (78 +/- 15% 60 min post-hypoxia; P < 0.05) with an associated increase in burst frequency (11 +/- 2.1 bursts min-1; P < 0.05). In contrast, 9 or 20 min continuous hypoxia did not elicit LTF of either phrenic amplitude (4.7 +/- 5.1 and 10.1 +/- 10.2% 60 min post-hypoxia, respectively; P > 0.05) or frequency (4.6 +/- 1.3 and 5.1 +/- 2 bursts min-1 60 min post-hypoxia, respectively; P > 0.05). 4. The results indicate that hypoxia-induced long-term facilitation of phrenic motor output is sensitive to the pattern of hypoxic exposure in anaesthetized rats.