This thesis describes experiments to measure the ventilatory response to hypoxia at a constant (isocapnic) level of CO2 (HVR) in 18 subjects. So as to provide a complete picture of the autonomic responses, middle cerebral artery velocity, a surrogate for cerebral blood flow (CBF), as well as finger plethysmography blood pressure (BP) were also measured. Ventilatory responses have been previously described only in terms of an acute peak followed by a decline. However, rather than a single type of response, I found four types categorized as: Decline, Double, Plateau, or No response. The Double pattern, characterized by a second peak of response was the most common, yet is described here for the first time. These patterns are also characteristic of the CBF and BP responses. Furthermore the temporal correlations between these brainstem-controlled responses are also reported here for the first time.