Fig. 3. Heart rate (f_H) profiles before, during and after voluntary dives in (A) -adrenergic- and ß-adrenergic-blocked harbour seals and in (B) muscarinic-, muscarinic- plus -adrenergic-, and muscarinic- plus ß-adrenergic-blocked seals. Control data for the two oral drugs (A) and for the three injected groups (B) were combined, although statistical analyses were performed on controls for each data set. Arrows denote the beginning and end of the dive. Each data point represents the mean f_H for the preceding 5 s interval. For each treatment, mean f_H during two dives (approximately 120 s) were averaged for each animal. Data from all seals (N=5) were then combined to give the means ± S.E.M. illustrated. The data were normalized so that dives of different lengths ended at the same time.