To determine the effects of preparatory phase training on aerobic parameters, resting heart rate variability (HRV) and 5-km performance of high-level endurance runners and the relationship between the percentage change (% change) of resting HRV with the % change of aerobic parameters and 5-km performance. Six runners were assessed before and after seven weeks of training. The aerobic parameters were determined in an incremental test. The HRV was assessed by a heart rate monitor. Athletes performed a 5-km running test in a track. The analysis revealed \'likely\' and \'very likely\' improvements for velocity associated with maximal oxygen uptake ([Formula: see text]O2max) (20.0+/-1.0 km.h(-1) to 21.2+/-0.6 km.h(-1)) and 5-km performance (18.0+/-0.4 km.h(-1) to 18.9+/-0.7 km.h(-1)), respectively, as well as \'likely\' decrease in high frequency (41.4+/-18.5 nu to 30.4+/-14.3 nu), and increase in low frequency (58.5+/-18.5 nu to 69.6+/-14.3 nu) band densities. The variation in the velocity associated with [Formula: see text]O2max showed the highest correlation with 5-km performance (r=0.95). The % change in the square root of the mean sum of the squared differences between R-R intervals and standard deviation 1 were highly correlated with variation in 5-km performance (r=0.69 and 0.66). Changes in the velocity associated with [Formula: see text]O2max and vagally mediated HRV were highly associated with 5-km running performance within the investigated team. These results have important implications because these parameters can be assessed longitudinally to monitor adaptation to training.