The role of the inspiratory and expiratory airflow through nostrils during the process of breathing in the reflex reciprocal congestion and decongestion of the nasal cycle was examined in this study. Air flow through each nostril was measured separately in male subjects before and after four types of breathing practices for 15 min each consisting of (1) inspiration through the patent nostril and expiration through the congested nostril, (2) inspiration through congested nostril and expiration through patent nostril, (3) inspiration through both nostrils and expiration through mouth and (4) inspiration through mouth and expiration through both nostrils. The breathing practices had no effect on the congested nostril but caused decrease in air flow through the patent nostril, indicating congestion. It is suggested that the inspiratory and expiratory air flow through the nostrils caused reflex congestion of the patent nostril. The stimuli arising from the nasal mucosa due to air flow during breathing may form the basis for the reflex reciprocal congestion and decongestion of nostrils in the nasal cycle.