BACKGROUND: Short-term exposure to fine particulate matter (PM(2.5)) has been associated with reduced heart rate variability (HRV), an established indicator of cardiac autonomic function, but it remains uncertain which specific constituents of PM(2.5) had key impacts. OBJECTIVE: To examine the short-term associations between various PM(2.5) constituents and HRV measures. METHODS: We conducted a retrospective panel study among 78 participants who received repeated 24-h electrocardiogram testing in Shanghai, China from 2015 to 2019. We obtained daily concentrations of 14 main chemical constituents of PM(2.5) from a fixed-site monitor. During 3 or 4 rounds of follow-ups, we measured 6 HRV parameters, including 3 frequency-domain parameters (power in very low frequency, low frequency and high frequency) and 3 time-domain parameters (standard deviation of normal-to-normal intervals, root mean square successive difference and percent of adjacent normal R-R intervals with a difference >/=50 msec). We used linear mixed-effects models to analyze the data after controlling for time trends, environmental and individual risk factors. RESULTS: The average daily PM(2.5) exposure was 45.8 mug/m(3) during the study period. The present-day exposure to PM(2.5) had the strongest negative influences on various HRV indicators. These associations attenuated greatly on lag 1 d or lag 2 d. Elemental carbon, organic carbon, nitrate, sulfate, arsenic, cadmium, chromium and nickel were consistently associated with reduced HRV parameters in both single-constituent models and constituent-PM(2.5) models. CONCLUSION: Our study highlighted the key roles of traffic-related components of PM(2.5) in inhibiting cardiac autonomic function.