We examined the effect of daily stress, age, and emotional stability/neuroticism on stress reactivity, using cortisol diurnal rhythms. We used data from the Normative Aging Study (Spiro & Bosse, 2001). The 72 men in this study ranged from 67-93 (M =79.29, SD =4.88). Multilevel modeling showed that higher daily stress predicted flatter cortisol diurnal rhythms, B = .09, p < .001, as did age, B = .01, p < .001, while those higher in neuroticism on the emotional stability/neuroticism measure showed steeper slopes for cortisol diurnal rhythms, B = -.04, p < .001. These results indicate that age and emotional stability/neuroticism levels explain some of the variance in individual differences in stress reactivity and provide a basis for future research focused on the effects of psychosocial variables on physiological outcomes. / Graduation date: 2013