PROJECT SUMMARY While melatonin is commonly thought of as a regulator of circadian rhythm, it has broad physiologic actions. Melatonin may regulate immune function and protect the host against toxicants. The parent grant aims to investigate melatonin as a nephroprotectant agent. The nephroprotective effects are likely mediated through binding to the MT1/MT2 receptors, activation of nuclear receptors, and receptor-independent mechanisms. The current supplement aims to evaluate racial differences in endogenous serum melatonin concentrations, mitochondrial density, and expression of MT1/MT2 in human kidneys. While data regarding plasma and urine melatonin concentrations in humans are available, few studies have evaluated tissue concentrations, and even fewer (if any) examined racial differences. Moreover, racial differences in mitochondrial density and MT1/MT2 receptor expression in human kidneys have not been previously examined. The kidneys have one of the highest densities of mitochondria in the human body and are essential for kidney health. Understanding biological differences may help identify why there are disparities in acute kidney injury.