This paper presents a study performed in 25 young healthy subjects measuring the evolution of heart rate variability (HRV) indices during emotional stress. Acute emotional stress was generated with a modified version of Trier Social Stress Test (TSST). The TSST comprises several tasks which include a memory test, anticipation of stress, public exposition, and an arithmetic task. Each task has different demanding conditions, carrying subjects' emotional stress to different states. An autogenic relaxation was done before TSST. Significant differences in HRV indices were observed in the arithmetic and memory task with respect to the relaxation stage. In particular during the arithmetic task, mean heart rate increased 22% (p-value <0.00001) the power in the very low frequency band increased 47% (p-value <0.00001 and normalized power in the low frequency (LF) band increased 19% (p-value <0.04). These results support a sympathetic activation during these tasks.