The 24-hour variation in physiological and psychological functioning within the individual and interpersonal differences were investigated in relation to the nature of social interaction between individuals. Level of activation, body temperature, and interpersonal conflict were studied in a sample of 16 married couples for a period of six weekdays of routine activity. Homogeneity was a major factor in sample selection. Body temperature was measured every hour during the waking time by electronic thermometer, and level of activation was measured four times a day on alternate forms of a self-report adjective checklist. Interpersonal conflict, defined as perceived fulfillment of emotional and interaction needs, was measured in the morning and late day. The Interpersonal Conflict Scale with established validity was constructed for the study and had two alternate forms of equal reliability. An index of desynchrony between partners for both temperature and activation was calculated for each day of data collection by the following method: The deviation score from the overall six-day mean was determined for each measurement time for each spouse and for each variable. Absolute values of the differences between deviation scores were added to obtain an index of desynchrony in that variable for the day. A daily mean of interpersonal conflict scores for both partners was also calculated. The hypotheses that a desynchrony between partners in body temperature rhythm and in level of activation rhythm would be positively related to conflict were tested by the Pearson product moment correlation. Obtained coefficients were not significant at the .05 level.