OBJECTIVES: To test for simvastatin-induced changes in affect and affective processes in elderly volunteers. DESIGN: Randomized, clinical trial. SETTING: The Geriatric Behavioral Psychopharmacology Laboratory at the University of Pennsylvania. PARTICIPANTS: Eighty older volunteers, average age 70, with high normal/mildly elevated serum cholesterol. INTERVENTION: Simvastatin up to 20 mg/d or placebo for 15 weeks. MEASUREMENTS: Daily diary records of positive and negative affects and of events and biweekly measures of depressive symptoms. Affect ratings were obtained using the Lawton positive and negative affect scales; independent raters coded the valences of events. RESULTS: Thirty-one of 39 subjects assigned to placebo and 33 of 41 receiving simvastatin completed the study. During biweekly assessments, four subjects on simvastatin and one on placebo experienced depressive symptoms, as manifest by Center for Epidemiological Studies Depression scale scores greater than 16 (exact P=.36). Diary data demonstrated significant effects on affective processes. For positive affect, there was a significant medication-by-time interaction that reflected decreases in positive affect in subjects receiving simvastatin, greatest in those patients whose final total cholesterol levels were below 148 mg/dL. For negative affect, there were significant medication-by-event, and medication-by-event-by-time interactions, reflecting a time-limited increase in the apparent effect of negative events. CONCLUSION: Simvastatin has statistically significant effects on affect and affective processes in elderly volunteers. The decrease in positive affect may be significant clinically and relevant to the quality of life of many patients.