Introduction: Carnitine is a dietary supplement available freely on the market. Despite the large amount of research there is virtually no data on its effect on autonomic heart rhythm control. Aim: The study aimed to test the effect of 24-week carnitine supplementation on HRV in healthy people of advanced age. Methods: Volunteers aged 64-74 (n=13) participated in the study. The individuals were randomly divided into two groups: one supplemented with carnitine (2g/day) and their control group supplemented with a placebo for 24 weeks. High resolution ECG (4 kHz; PowerLab, AdInstruments, Australia) was recorded before and after the supplementation. Five-minute time-series of NN intervals were used for calculation of HRV parameters using Kubios Premium 3.1 software (Kuopio, Finland). Time- and frequency-domain (FFT method), Poincare plot and nonlinear parameters were assessed. Results: Supplementation with carnitine affected parasympathetic modulation of autonomic control of the heart. Surprisingly HRV parameters of vagal drive were decreased in carnitine group: rMSSD (from 23.87±8.84ms to 14.35±4.72ms; p=0.034), SD1 (from 16.91±6.26ms to 10.16±3.34ms, p=0.034) and HF (from 155.6±88.53ms2 to 79.38±52.66ms2, p=0.049). In contrast all indexes of long-term variability remained unchanged: SDNN 21.91±5.59 vs. 17.34±7.98ms, p=0.156; SD2 25.68±6.52 vs. 22.29±10.91ms, p=0.313; LF 202.3±127.6 vs. 183.7±171.9ms2, p=0.706. Interestingly, indexes of autonomic balance were almost unchanged: LF/HF 1.85±1.63 vs. 2.46±1.06, p=0.400 and SDNN/rMSSD 0.99±0.26 vs. 1.21±0.21, p=0.078. In placebo group all parameters remained unaltered. Additionally, nonlinear indexes such as: sample entropy, approximate entropy, detrended fluctuation analysis scaling exponent alfa1 and alfa2 and recurrence rate also remained unaffected in both groups. Summary: In people with advanced age carnitine supplementation suppressed vagal influence on the heart rhythm. Such profile of autonomic dysregulation is unbeneficial and may increase severe cardiac incidents.