Force dived animals undergo cardiovascular changes characterized by bradycardia, increased total peripheral resistance, and changes in blood flow distribution. Since these changes occur in decerebrated animals, the dive response must be a brainstem reflex. However, in voluntary dives, animals may show anticipatory bradycardia and may also adjust their cardiovascular responses according to anticipated dive duration, indicating suprabulbar influences upon dive responses. Studies of heart rate using telemetry have shown that there can be substantial differences in the dive response of voluntarily and force dived animals. Furthermore, some animals show a "fear bradycardia" when trapped in a stressful situation, leading some researchers to suggest that bradycardia during forced submersion is an artifact of the stress of the situation. Muskrats (Ondatra zibethica) were observed freely diving for food in an indoor tank using a video camera and VCR unit. EKG was telemetered from the animals and recorded on the audio channel of the VCR tape. Heart rate responses to voluntary dives were analyzed and compared with those from escape and forced dives. Heart rate responses were also recorded from decorticate and sham operated muskrats to elucidate the role that the cerebral cortex plays in the dive response. In all types of dives, muskrats exhibited a rapid and large bradycardia upon submergence (heart rate declined by greater than 55% of the predive heart rate). Obviously diving bradycardia in the muskrat was not due to fear or stress, but occurred as a response to submersion per se. There was no evidence of post-dive tachycardia or anticipatory immersion bradycardia. Disturbing the animal in a non-diving situation resulted in only a 13% decrease in heart rate. In intact animals voluntary, escape, and forced submergence resulted in progressively greater decreases in heart rate. Heart rate fell by 56% in voluntary dives, 65% in escape dives, and 73% in forced dives. Intensification of the bradycardia to a lower heart rate than that seen in voluntary dives was mediated by the cerebral cortex, as heart rate in decorticate muskrats in escape and forced dives did not fall below that seen in voluntary dives. This indicates that the final adjustment of dive heart rate is dependent upon an intact cerebral cortex. However, in decorticate muskrats there appeared to be a recovery of cortical function, as intensification of bradycardia in forced dives was dependent upon the time that had elapsed after surgery. This study shows that there is a cortical influence upon the cardiovascular system during diving. It also indicates that in experiments with unanesthetized animals, the degree of stress of the situation must be taken into account, as this may affect physiological responses. / Science, Faculty of / Zoology, Department of / Graduate