The relationship between aerobic training, vagal influence on the heart and ageing was examined by assessing aerobic fitness and resting heart rate variability in trained and untrained older men. Subjects were 11 trained cyclists and runners (mean age = 66 +/- 1.6 years) and 11 untrained, age-matched men (mean age = 66 +/- 1.2 years). Heart rate variability testing involved subjects lying supine for 25 min during which subjects\' breathing was paced and monitored (7.5 breaths min(-1)). Heart rate variability was assessed through time series analysis (HRV(ts)) of the interbeat interval. Results indicated that trained older men (355 +/- 0.21 1 min(-1)) had significantly (P < 0.05) greater VO2(max) than that of control subjects (2.35 +/- 0.15 1 min(-1)). Also, trained older men (52 +/- 1.8 beats min(-1)) had significantly (P < 0.05) lower supine resting heart rate than that of control subjects (65 +/- 4.2 beats min(-1)). HRV(ts) at high frequencies was greater for trained men (5.98 +/- 0.22) than for untrained men (5.23 +/- 0.32). These data suggest that regular aerobic exercise in older men is associated with greater levels of HRV(ts) at rest.