In humans, increased body weight and arterial stiffness are significantly associated, independently of blood pressure (BP) level. The finding was never investigated in rodents devoid of metabolic disorders as spontaneously hypertensive rats (SHR). Using simultaneous catheterization of proximal and distal aorta, body weight, intra-arterial BP, heart rate (HR), and their variability (spectral analysis), aortic pulse wave velocity (PWV) and systolic and pulse pressure (PP) amplifications were measured in unrestrained conscious Wistar-Kyoto (WKY) rats and SHR between 6 and 24 weeks of age. Aortic proximal systolic, diastolic, pulse and mean BP were significantly higher in SHR than in WKY rats and increased significantly with age (at the exception of PP). PP amplification increased with age but did not differ between strains. PWV was significantly associated with HR variability. Based on two ways variance analysis, PWV was significantly higher in SHR than in WKY rats (strain effect) and increased markedly with age in both strains (age effect). Adjustment of PWV to mean BP attenuated markedly both the age and the strain effects. After adjustment to body weight, either alone or associated to mean BP, the age effect was no more significant, but the strain effect was markedly enhanced. In conscious unanesthetized SHR and WKY rats, aortic stiffness is consistently associated with body weight independently of age and mean BP. An intervention study should have to consider in the objectives: SBP and PP amplification measured in conscious animals, central control of body weight and autonomic nervous system.