Examples of minute ventilation responses to hypercapnia (ramp increase; 0 to 5% inspired carbon dioxide (CO₂) in 2 min) in two 20-day-old opioid-naive rats (similar weight), including one tested during natural quiet rest (no morphine) and the other tested under morphine sedation 30 min after 5 mg/kg subcutaneous administration of morphine. Also shown are equations that describe the best-fit linear solutions of the data. When compared with the rat resting quietly without morphine, minute ventilation was substantially depressed (−46%) before exposure to carbon dioxide in the morphine-treated rat (see 0% CO₂), and the normal response of increased minute ventilation in response to increased carbon dioxide was essentially eliminated.