1. The age-related changes in mean arterial pressure (MAP), heart rate (HR), and baroreflex sensitivity (BRS) were studied in spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats from 4 to 20 weeks of age. 2. Intra-arterial blood pressure (BP) was continuously recorded for 24 h in conscious, freely moving rats. Twenty-four hour MAP and HR were calculated by an online computer. Baroreflex sensitivity was measured by phenylephrine infusion. 3. In SHR, BRS was significantly lower than in WKY as early as 4-5 weeks, at which time MAP in SHR was only slightly raised. During subsequent weeks, rapid increase in MAP occurred in SHR, in association with progressive bradycardia. 4. It was concluded that a reduced BRS may be detected in young prehypertensive SHR and this impairment of BRS may be central in origin.