Underwater feeding behavior was measured in 10 captive canvasback ducks (Aythya valisineria) for 12 days under a 12:12-h light-dark photoperiod. Feeding activity exhibited a daily rhythm, with 76% of dives occurring at night. In separate experiments on six of these ducks, a circadian rhythm was observed in the duration of voluntary dives. Dives at night (14.7 ± 0.7 s) were significantly longer than those during the day (10.7 ± 0.7 s). These day-night differences in diving behavior were accompanied by day-night differences in respiratory responses to progressive asphyxia. In the same six ducks, ventilation increased exponentially as a function of inspired CO₂ concentration during rebreathing in a closed-circuit barometric plethysmograph. The exponential rate constant for inspired ventilation was significantly smaller at night (0.23 ± 0.02) than during the day (0.26 ± 0.01). We suggest that intermittent apneic exercise is facilitated by reduced respiratory chemosensitivity and that the respiratory and behavioral control systems are synchronized by the circadian timing system in diving ducks.