Cerebral vasodilator responses to progressive increases in hypercapnia are attenuated with age, diabetes, and hypertension (1, 2, 3). In addition, such vasodilator responses are reduced following cyclooxygenase (COX) inhibition (using indomethacin). Similarly, cerebral vasoconstrictor responses to progressive decreases in arterial CO2 (during the transition from hypercapnia to eucapnia) in young adults were blunted after indomethacin (4). However, it is unknown whether cerebral vasoconstrictor responses during this recovery are altered by aging, or whether COX inhibition alters this recovery response in older individuals. Twenty-one young (n=11; 26 ± 5y) and old (n=10; 65 ± 6y) healthy adults participated in the study. Subjects were fitted for a mask and breathed 6% CO2 for 3 minutes and then returned to breathing room air before (Pre) and 90 minutes after (Post) indomethacin administration (1.2mg/kg). Middle cerebral artery velocity (MCAv), cerebrovascular conductance index (CVCi), ventilation (VE) and end-tidal CO2 (ETCO2) were averaged for each breath during recovery from hypercapnia. Values are means ± S.E.M., compared by a two-way repeated measures ANOVA. The change in MCAv during the transition from hypercapnia to eucapnia (Δ MCAv) was greater in young compared with old subjects (36 ± 4 vs. 23 ± 3 cm/s; p<0.01) at Pre, and significantly blunted in both young (16 ± 2 cm/s) and old (11 ± 2 cm/s) by indomethacin. There were no group differences in the change in ETCO2 (Δ ETCO2) from hypercapnia to eucapnia, however, Δ ETCO2 was greater after indomethacin in both young (17 ± 1 vs. 20 ± 1 mmHg; p<0.01) and old (15 ± 1 vs. 18 ± 1 mmHg; p<0.05) subjects. MCAv and CVCi responsiveness to CO2 during recovery from hypercapnia was not different between young and old subjects. However, MCAv responsiveness was reduced by indomethacin in both young and old adults (58% and 70% of Pre, respectively; p<0.001). There were no changes in VE sensitivity between groups or between Pre and Post. Taken together, this data demonstrates that cerebral vasoconstrictor responses to reductions in CO2 are preserved in healthy aging. In addition, COX inhibition blunted these vasoconstrictor responses, suggesting that prostaglandins have a role in regulating cerebrovascular tone in both young and old adults.