The evolution with age of blood pressure (BP), heart rate (HR), and baroreflex sensitivity was studied in hypertensive (LH), normotensive (LN), and low blood pressure (LL) male rats of the Lyon strains. Intra-arterial BP was continuously recorded for 48 h in freely moving animals. On-line computer analysis of the BP curve allowed calculation of systolic (SBP), mean (MAP), and diastolic (DBP) blood pressure, HR, and their variability. At 5 wk of age, LH rats exhibited a higher BP than LN and LL controls, which, at 21 wk of age, was associated with significant bradycardia and a greater variability in DBP and HR. LL rats had a MAP similar to LN, which confirms that both strains are, in terms of BP, valid controls for LH animals. The baroreflex sensitivity measured after phenylephrine injections increased with age more markedly in LN and LL than in LH rats, whereas in the latter, it was significantly lower at the age of 9 and 21 wk. When measured after nitroglycerin injections, the baroreflex sensitivity was higher and more stable than that observed after phenylephrine administration. In addition, the baroreflex sensitivity in the different strains was inversely related to the level and the variability of DBP but not of HR.