Ascending aortic input impedance spectra and pressure augmentation index was determined in young (8-month) and old (29-month) mice to see if arterial stiffening occurred with age in mice as it does in humans. Pressure and blood velocity signals measured simultaneously from the same location in the ascending aorta of a mouse were used to determine input impedance spectra (DC-10 harmonics). The first minimum of the impedance modulus shifted from the 2nd harmonic in young mice to the 4th harmonic in old mice. Characteristic impedance (estimated as average of 2nd to 10th harmonic) was significantly higher in old mice (mean/spl plusmn/SEM, 471/spl plusmn/62 vs. 299110 dynes-s/cm/sup 3/, p<0.05). Pulse pressure and augmentation index, determined from the aortic pressure signals were also higher in old mice: pulse pressure (42/spl plusmn/2.2 vs. 29/spl plusmn/4.9 mmHg, p<0.05) and augmentation index (37/spl plusmn/5 vs. 14/spl plusmn/2 %, p<0.005). These results are similar to the findings reported in humans and confirm that mice may be used as models of age-related vascular stiffening.