HEART rate is usually reduced when aquatic mammals dive. The degree of this reduction, known as bradycardia, is remarkable1–3, and immersion in water has been regarded as important in its initiation3,4. The heart of the phocid seal beats slower in forced or restrained dives than during trained or voluntary dives5,6, and bradycardia occurs during respiratory pauses in air as well as in water7–9. Central nervous system (CNS) control of bradycardia has been suggested5,6,8,9. We have investigated whether bradycardia can be conditioned in the California sea lion Zalophus californianus since cardiovascular responses can be conditioned in man and many other animals10.