Ventilatory parameters such as tidal volume, minute ventilation (VE), and inspiratory flow rate decrease in 24- vs. 12-month-old male and female rats. Differences between male and female values are maintained with age. Ventilatory response male and female rats exhibit to hypercapnia is altered by a decrease of the intercept, but not the slope value. The pattern of breathing exhibited by young females and males in response to hypercapnia (i.e. an increase of VT and f) is different than that noted in old males and females (i.e. an increase in VT only). In contrast, the ventilatory response both of slope and intercept male rats show by 24 months is decreased compared to the 12-month value; but the slope value is actually increased in the older vs. younger female rats in response to hypoxia.