Fig. 2. Mean (±s.e.m.) instantaneous heart rate for 21 observations on seven jeju, as calculated per R–R interval over 20 heartbeats prior to and succeeding an air breath (see text for details). Each panel shows the fish under (A) hypoxia, (B) hypoxia following adrenergic receptor blockade, and (C) hypoxia following adrenergic and cholinergic receptor blockade. The black arrow denotes when exhalation was completed and inhalation initiated.