Heart rate and breathing movements have been monitored in three unrestrained harbor seals. On voluntary submergence heart rate fell markedly in all seals, and after 2–3 s submergence stabilized at about 40–50% of the predive level. Heart rate increased before the animals broke surface at the end of the dive and, when breathing began again, a postdive tachycardia was observed. Two of the three seals frequently showed anticipation of the dive as judged from their heart-rate response. Heart rate during feeding dives was generally more variable; in fact one seal exhibited no bradycardia in 20% of its feeding dives, although another seal showed a significantly greater bradycardia than was seen in routine dives. When breathing rate was low (less than five breaths min−1) respiratory variations in the heart rate occurred, although the onset of bradycardia was much more rapid during diving than during breath-holding. Because of the flexibility of the response it is concluded that the generation of diving bradycardia in the seal is a complex phenomenon which, aside from any responses set in train by peripheral receptors, must also involve some form of associative learning.