Parkinson's disease is known to affect the reflex cardiovascular control systems, resulting in a suppressed heart rate variability, but present knowledge concerning the long-term characteristics of heart rate and heart rate variability, e. g. circadian regulation, is limited. We investigated the circadian fluctuation of the time domain, frequency domain and some non-linear measures of heart rate variability in 44 untreated patients with Parkinson's disease and 43 age- and sex-matched control subjects.In the parkinsonian patients, the measured power spectral components of heart rate variability (low-frequency power and high-frequency power) and the SD(1) value of the Poincaré two dimensional vector analysis, that quantifies the short term beat-to-beat variability, were suppressed at night. During the daytime only the SD1 of the Poincaré was suppressed. The night-to-day-ratios of the heart rate variability measures did not differ significantly between the patients and the controls. The results indicate that the long-term parasympathetic cardiovascular regulation is impaired in untreated patients with Parkinson's disease. The dysfunction is more pronounced at night.