Group differences have been seen in experimental pain tests and hormone levels are often estimated based on biological factors, such as age. To date, it is unknown how circulating hormone levels affect acute pain sensitivity and, given the rise in hormonal supplement usage (i.e., for low testosterone or menopause) and errors in age-based estimations, these data may be critical to assessing potential risk for chronic pain. Further, while we have shown that immune cells are critical to the persistence of chronic pain in animals, it is unclear how circulating hormone levels affect (a) these cell populations in healthy adults, (b) acute pain sensitivity in humans and (c) other psychological factors that may contribute to pain sensitivity. The current project is focused on examining the role that circulating hormones may play in various aspects that contribute to the susceptibility to chronic pain. Specific aims of the study include: 1. To determine the impact of circulating hormone level on pain sensitivity. 2. To quantify differences in immune cell populations and activity. 3. To examine social and psychological factors that contribute to pain sensitivity and the impact of hormones. We will recruit healthy individuals between the ages of 18 and 65 years of age. We will use quantitative sensory testing to assess sensitivity to cold, pressure, and heat via standardized protocols. Blood samples will be taken for assessment of circulating hormone levels (testosterone, estradiol, cortisol), immune cell populations and stimulated cytokine release, as well as salivary samples to measure stress reactivity. Questionnaires will measure pain state, quality of life, self-reported health, sleep, depression, and social support and activity/sleep monitors will provide objective measures of sleep. Together, this will be the first exploration of the impact of circulating hormones on factors that may predispose individuals to chronic pain, carried out in a broad population of healthy adults.