Indices of heart rate variability (HRV) reflect cardiac autonomic tone and may be markedly affected by pheochromocytoma. The effect of pheochromocytoma on HRV was determined by Holter monitoring before diagnosis, under pre-operative alpha-blockade and 5 and 19 months after surgery in a 40 year-old female. Mean heart rates, although higher under alpha-blockade, were unchanged by surgery but indices of HRV reflecting both short term (vagally mediated) and longer term (mediated by vagal, sympathetic and other influences) rhythms were diminished under alpha-blockade and post-surgery. High frequency power (0.15-0.40 Hz), an index of vagal tone, declined from 512 ms2 pre-diagnosis to 220 ms2 under alpha-blockade to just over 100 ms2 post-surgery. Low frequency power (0.04-0.15 Hz), a measure reflecting both vagal and sympathetic tone, declined from 409 ms2 pre-diagnosis to 186 ms2 under alpha-blockade and was just over 200 ms2 post-surgery. SDNN, the standard deviation of normal-to-normal interbeat intervals over 24 hours, declined from 118 ms pre-diagnosis to just over 70 ms both under alpha-blockade and post-surgery. The ratio of low frequency to high frequency power (LF/HF ratio) increased to 0.84 under alpha-blockade, and doubled after surgery (0.79; before, 2.05; after). These changes in HRV may provide insights into the effects of endogenous catecholamines and intrinsic counter-regulatory autonomic mechanisms on HRV.