Conventional spectral analyses of heart rate variability (HRV) have been limited to stationary signals and have not allowed the obtainment of information during transient autonomic cardiac responses. In the present study, we evaluated the ability of the short-time Fourier transform (STFT) method to detect transient changes in vagal effects on the heart. We derived high frequency power (HFP, 0.20 - 0.40 Hz) as a function of time during active orthostatic task (AOT) from the sitting to standing posture prior to and after selective vagal (atropine sulphate 0.04 mg/kg) and sympathetic (metoprolol 0.20 mg/kg) blockades. The HFP minimum point during the first 30 s after standing up was calculated and compared with sitting and standing values. Reactivity scores describing the fast and slow HFP responses to AOT were calculated by subtracting the minimum and standing values from the sitting value, respectively. The present results, obtained without controlled respiration, showed that in the drug-free condition, HFP decreased immediately after standing up (p<0.001) and then gradually increased towards the level characteristic for the standing posture (p<0.001), remaining lower than in the sitting baseline posture (p<0.001). The magnitudes of the fast and slow HFP responses to AOT were abolished by the vagal blockade (p<0.001) and unaffected by the sympathetic blockade. These findings indicate that HFP derived by the STFT method provided a tool for monitoring the magnitude and time-course of transient changes in vagal effects on the heart, without the need to interfere with normal control by using blocking drugs.