We compared the effects of intravenous morphine (5 mg·kg⁻¹) and fentanyl, (50μg·kg⁻¹) on systolic blood pressure (SBP), heart rate (HR), and efferent sympathetic adrenal nerve action potentials (SANA) in rats. We also determined the extent of the reflex responses of these parameters of 9% carbon dioxide (CO₂) challenge during the above narcotic anesthesia. In the morphine group, SBP was elevated and the elevated levels were maintained, while changes in SBP in the fentanyl group were not significant. In the morphine group, SANA showed initial stimulation and subsequent depression, while in the fentanyl group, SANA showed sustained depression. CO₂ challenge induced only very small changes in SBP and HR, suggesting that during high-dose narcotic anesthesia the hypercapnic stimulus may not be reflected in circulatory parameters. In both groups, hypercapnia increased SANA to 30% of the baseline values from the pre-challenge level. However, these values were only 91% and 56% of the baseline value in the morphine and the fentanyl, groups, respectively.