Sixteen healthy volunteers breathed 100% O2 or room air for 10 min in random order, then their ventilatory response to sustained normocapnic hypoxia (80% arterial O2 saturation, as measured with a pulse oximeter) was studied for 20 min. In addition, to detect agents possibly responsible for the respiratory changes, blood plasma of 10 of the 16 subjects was chemically analyzed. 1) Preliminary O2 breathing uniformly and substantially augmented hypoxic ventilatory responses. 2) However, the profile of ventilatory response in terms of relative magnitude, i.e., biphasic hypoxic ventilatory depression, remained nearly unchanged. 3) Augmented ventilatory increment by prior O2 breathing was significantly correlated with increment in the plasma glutamine level. We conclude that preliminary O2 administration enhances hypoxic ventilatory response without affecting the biphasic response pattern and speculate that the excitatory amino acid neurotransmitter glutamate, possibly derived from augmented glutamine, may, at least in part, play a role in this ventilatory enhancement.