Effects of chronic hypoxia and an NMDA receptor antagonist (MK801) on (A) TI, (B) TE and (C) VT/TI for CON and CSH rats before and after microinjection of MK801. Data are means ± s.e.m. from n = 13 CON and 8 CSH rats. *Significant difference between CON and CSH values. †Significant difference between artificial cerebral spinal fluid and drug injections. ‡No significant difference between 10% and 21% O2 (P < 0.05). CON, controls; CSH, chronic sustained hypoxia; TE, expiratory time; TI, inspiratory time; VT/TI, ventilatory drive to breathe.