ABSTRACT Cardiovascular chemoreceptors are crucial for the development of forced diving responses in dabbling ducks such as the mallard (Anas platyrhynchos). Denervation of the peripheral chemoreceptors or allowing the duck to breath 100 % oxygen before a forced dive eliminates or considerably delays the onset of noticeable bradycardia (Jones & Purves, 1970; Mangalam & Jones, 1984). These procedures have no effect on the level of bradycardia established after 10 s or so underwater in diving ducks or cormorants in forced dives (Butler & Woakes, 1982; Mangalam & Jones, 1984; Furilla & Jones, 1986). The afferent neural pathway for the dive response is unclear in cormorants, but in diving ducks it is the stimulation of nasal receptors with water that causes dive bradycardia (Furilla & Jones, 1986). When forced dives are prolonged, then peripheral chemoreceptors appear to affect heart rate in diving ducks. Heart rate is elevated in ducks (Aythya fuligula) in which the carotid body has been denervated, compared with intact ducks, after 50 s or so of forced submergence (Butler & Woakes, 1982). The purpose of the present study was to investigate whether chemoreflexes also influence heart rate in cormorants in forced dives of up to 120s duration.