Prior lab research revealed higher basal total peripheral resistance (TPR) and lower cardiac output (CO) in lonely than in nonlonely young adults. In this study, experience sampling was used to obtain ambulatory blood pressure; impedance cardiography: and reports of activities, appraisals, interactions, and health behaviors. Results confirmed that loneliness predicted higher TPR and lower CO during a normal day. Loneliness did not predict differences in time spent alone, daily activities, or health behaviors but did predict higher stress appraisals and poorer social interactions. Independent of loneliness, interaction quality contributed to TPR. Loneliness differences were not mediated by depressed affect or neuroticism. Social support mediated loneliness differences in stress and threat. Concomitants of loneliness were comparable for men and women.