Long term facilitation (LTF) is a serotonin-dependent augmentation of respiratory motor output (phrenic and hypoglossal) following episodic hypoxia. Since ageing influences respiratory control mechanisms and serotonergic function, we tested the hypothesis that LTF decreases with age in male rats. Young (3-4 month) and aged (13 month) male Sprague-Dawley rats were anaesthetized with urethane, vagotomized, paralysed and pump ventilated. Integrated phrenic and hypoglossal (XII) nerve activities were measured before (baseline), during and for 60 min after three 5 min episodes of isocapnic hypoxia (Pa,O2 35-45 mmHg) separated by 5 min of hyperoxia (Pa,O2 > 150 mmHg). In young rats, LTF was observed as an augmentation in peak integrated phrenic (n = 8) and XII (n = 7) amplitudes following episodic hypoxia (56 +/- 14 and 73 +/- 16 % (means +/- S.E.M.) at 60 min post-hypoxia, respectively; both P < 0.05). In aged rats, LTF was significantly increased compared to baseline in phrenic (25 +/- 8 % at 60 min, P < 0.05), but not in XII (4 +/- 7 %, P > 0.05) motor output. LTF was significantly greater in young than in aged rats in both motor outputs (P < 0.05). Decreased phrenic and XII LTF suggests that serotonergic modulation of respiratory motor output decreases in ageing male rats. We speculate that decreased serotonergic modulation may contribute to age-related breathing disorders.