Is the white coat effect an alert reaction? In this cross-sectional study we compared the white coat effect on systolic blood pressure with the systolic blood pressure reactivity obtained during a stress test. The influence of the sympathetic system (LF band of systolic BP) and the parasympathetic system (HF band of pulse rate) on white coat systolic blood pressure and stress test systolic blood pressure were analysed. We stratified 174 subjects into two groups, according to their blood pressure: hypertensives (HT, n=44, BP>140/90 mmHg) and normotensives (NT, n=130). The BP was recorded during an occupational health consultation, over 24 hours, and beat to beat during a stress test (Finapress). White coat systolic BP was calculated as the difference between the consultation BP and the average systolic BP over 24 hours. The white coat systolic BP was not related with an increase in pulse rate. In contrast, during the stress test the increases in systolic BP and pulse rate were correlated (r=0.44; p<0.001). The white coat systolic BP was lower than the stress test systolic BP in the NT (6.6 +/- 7.2 vs 23 +/- 12 mmHg; p<0.001) and in the HT (16 +/- 11 vs 29 +/- 17 mmHg; p<0.001). The HT had a lower parasympathetic index than the NT (0.45 +/- 0.43 vs 0.92 +/- 0.83 bpm2; p<0.001). In the HT the white coat systolic BP was positively correlated with the stress test systolic BP (r=0.47: p<0.01) and negatively with the parasympathetic activity index. In conclusion, for recently diagnosed and untreated HT an early alteration of the parasympathetic system reveals that the white coat effect is a low amplitude alert reaction.