Various time- and frequency-based approaches have been proposed to measure baroreflex sensitivity. All these methods are block-based ones. This study proposes a novel instantaneous measure of baroreflex sensitivity that enables the investigation of frequency domain characteristics of baroreflex sensitivity. This study was performed using the EUROBAVAR dataset, which consists of 10-12 minute recordings (in both supine and standing positions), acquired from 21 patients (age: 20-68 yrs.; 4 men). The data was obtained non-invasively from the subjects (healthy subjects and unhealthy ones with problems such as hypertension, heart transplant, and diabetes), and consists of the beat-to-beat interval, systolic, diastolic and mean blood pressure values. Instantaneous baroreflex sensitivity was obtained for each subject using a 15- second sliding window, by analyzing the ration between standard deviation of the inter-beat intervals and systolic blood pressure. Frequency characteristics of the instantaneous baroreflex sensitivity was then evaluated using power spectrum analysis. Power spectrum analysis indicated that the percentage of total power of the instantaneous baroreflex sensitivity was concentrated more in the very-low frequency band [0.003 Hz - 0.04 Hz], than the ultra-low frequency band [0.0 Hz - 0.003 Hz], (respectively 71±9 and 15±10%). The average peak frequency was 0.0193±0.008 Hz confirming the existence of an oscillation in the very-low frequency band. Furthermore, significant difference was found between the healthy and unhealthy populations. The proposed instantaneous baroreflex sensitivity measure is simply defined and computed. Results suggest that baroreflex sensitivity presents quasi-periodic fluctuations.