The effects of progressive isocapnic hypoxia on the systolic time intervals were studied in 10 healthy human subjects. We induced hypoxia by a rebreathing method and monitored the arterial oxygen saturation continuously and non-invasively by means of an ear oximeter. Arterial oxygen saturation (SaO2) was allowed to fall to a level of 75 per cent and was then held constant for five minutes. As SaO2 fell, heart rate increased linearly, with a mean increase of 0.83 beats/min per one per cent fall in SaO2. The pre-ejection phase index decreased from a mean of 127.2 ms at full oxygen saturation to 120.1 ms at steady-state hypoxia levels, while the ratio of the pre-ejection phase to left ventricular ejection time decreased from a mean of 0.330 to 0.301. The left ventricular ejection time index increased from 417.4 ms to 429.3 ms, while no statistically significant difference was found in the length of electromechanical systole.