Heart rate was telemetered from 6 preschool children during play sessions with their mothers. Their behavioral interaction was simultaneously recorded on videotape and rated on three dimensions of interaction: status (submission-dominance), affect (hostility-warmth), and degree of involvement; 100 specific behaviors were coded in consecutive 4-second epochs. In exploring the relationship between heart rate and the behavioral measures, we applied two kinds of analysis—state analysis and transition analysis. The usefulness of recording heart rate in a naturalistic setting was demonstrated by replicating the finding from more rigidly defined experiments that intense looking at an object is associated with cardiac deceleration. New findings were that submissive status and warm affect of the child and dominant status and warm affect of the mother were associated with low heart rate in the child and that the onset of smiling was associated with cardiac deceleration in most situations. The study demonstrates the feasibility and some of the potential uses of continuously telemetered heart rate for analyzing interactional and physiological variables in a naturalistic setting.