We evaluated pressor responses to the hyperventilation test in elderly normotensive (n=43, mean age 82 ± 5 years) and elderly hypertensive subjects (n=45 with essential hypertension, mean age 82 ± 2 years, and n=49 with secondary hypertension, mean age 82 ± 3 years). Hyperventilation did not change blood pressure (BP) in normotensive and secondary hypertensive subjects, whereas it decreased BP in essential hypertensives. Hierarchical cluster analysis based on BP responses to hyperventilation disclosed three groups of subjects in each population: group 1 exhibited a reduction in BP (essential hypertensives: 76%), group 2 no change (normotensives: 70%, secondary hypertensives: 76%), and group 3 an increase (normotensives: 19%, essential hypertensives: 13%, secondary hypertensives: 14%). Ambulatory BP monitoring found significant differences in pressor daytime profiles of hypertensive patients according to pressor responses to hyperventilation showing wide fluctuations in group 1 and 3 patients. Interestingly, the peak ambulatory SBP values correlated to the pre-hyperventilation SBP values in group 1, and to the hyperventilation peak SBP values in group 3. In conclusion: 1) Aging decreases reactivity to respiratory alkalosis in elderly normotensives; 2) hyperventilation induces significant pressor changes frequently in essential hypertension, but rarely in secondary hypertension; 3) the significant pressor responses to hyperventilation reflect the daytime pressor profiles predicting the highest daily fluctuations of BP values.