In normal humans, both hypoxia and hypercapnia result in sympathetic nerve activation, and when combined, i.e. hypoxic hypercapnia, synergistically increase sympathetic activity. Apnea during the hypoxic and hypercapnic stress results in further increases in sympathetic activity. Borderline hypertensive humans have exaggerated sympathetic nerve responses to hypoxia. Hypertensives are also prone to sleep apnea. We suggest that sleep apnea may result in very high levels of sympathetic activity which may contribute to daytime hypertension and/or precipitate cardiovascular catastrophe in hypertensive people during sleep.