Effects of face immersion on the cardiovascular system and the factors affecting the diving reflex were examined in 15 human subjects. During face immersion with breath holding the cardiac index decreased 22% (P< 0.001) and stroke index decreased about 14% (P > 0.05). The former decrease was accompanied by a significant decrease in heart rate. Brachial blood pressure increased more during face immersion with breath holding in cold (10-17 C) or slightly cool (30-37 C) water than it did during simple breath holding or application of an ice bag to the face. An analysis of pressure-wave contours was suggestive that peripheral vasoconstriction occurred most markedly during cold-water immersion. Heart rate was lowest during breath holding with face immersion in cold water irrespective of moderate changes in esophageal pressure and lung volume. Inhalation of asphyxic gases suggested that asphyxia is not a major factor in evoking the diving reflex. Cold receptors in the face may be the most effective trigger. Hypercapnia, however, followed by hypoxia during breath holding may contribute to the maintenance of the diving bradycardia.