In the attempt to delimit the conditions under which heart rate can be used to assess mediating events in the learning process, the relationships among heart rate, instrumental behavior and general activity in several conditioning situations are being evaluated. First, the contributions of associative and nonassociative factors to changes in activity and heart rate responses are being studied as a function of initial training-retest interval. Second, an analysis of heart rate and activity changes in terms of orienting and defensive responses will contribute to an understanding of the well-known stimulus effects in avoidance learning. Third, the relationships among heart rate, general motor activity and response suppression are being studied in a situation which provides slow acquisition and subsequent temporal discrimination of response suppression. Finally, the use of several pure strains and crossbred offspring which differ in heart rate baseline and responsiveness to shock will be used in the above studies and will provide data relevant to the main objectives as well as to an understanding of genetic determinants of behavior.