The purpose of this study is to characterize the heart rate variability (HRV) signal in the frequency domain by using wavelet and cosine packets. Here, the authors introduce the adapted wavelet transform methods to analyze heart-rate fluctuations. These methods were chosen because the components in the signals can be analyzed and quantified at different scales, e.g., long windows can be used for the low-frequency component while short-time windows can be used for high-frequency components. In this study, adaptive wavelet transform methods are applied to the HRV obtained from normal subjects. A comparison is made on the same data base with results based on the short-term Fourier transform method.