Two studies assessed the circadian variation of cardiovascular responses to stress in healthy and coronary artery disease (CAD) populations. In within-subjects designs, stressors were administered to healthy male subjects and male CAD patients both in the morning and afternoon, and subjects were classified as either morning or evening types using the Morningness-Eveningness Questionnaire (Horne & Ostberg, 1976, International Journal of Chronobiology, 4, 97-110). No consistent circadian variation in blood pressure or heart rate responses was observed in the aggregate sample of either healthy subjects or CAD patients. However, there were significant interactions between circadian type and time of day. In both populations, morning subjects exhibited higher cardiovascular levels during the morning session, and evening subjects exhibited higher levels during the afternoon session. Analyses of cardiovascular reactivity revealed less consistent evidence for this interaction. Self-reports of stress revealed interactions between time of day and morningness/eveningness only in the CAD sample. In CAD patients, preliminary analysis of myocardial wall function, an index of myocardial ischemia, did not reveal a significant interaction between morningness/eveningness and time of day, perhaps due to small sample size. The presence of differing circadian patterns in stress response based on individual differences in morningness/eveningness is discussed in terms of its methodological implications for psychophysiological research and in terms of the role of stress as an acute trigger of CAD.