Melanin concentrating hormone (MCH) is a hypothalamic neuropeptide with a well-characterised role in energy homeostasis and emergent roles in diverse physiologic functions such as arousal, mood and reproduction. Work to date has predominantly focused on its hypothalamic functions using animal models however little attention has been paid to its role in circulation in humans. The aims of this study were to a) develop a radioimmunoassay for the detection of MCH in human plasma; b) establish reference ranges for circulating MCH; and c) characterize the pattern of expression of circulating MCH in humans. A sensitive and specific RIA was developed and cross-validated by RP-HPLC and MS. The effective range was 19.5-1248 pg MCH/ml. Blood samples from 231 subjects were taken to establish a reference range of 19.5-55.4 pg/ml for fasting MCH concentrations. There were no significant differences between male and female fasting MCH concentrations however there were correlations between MCH concentrations and BMI in males and females with excess fat (p<0.001 and p=0.020) and between MCH concentrations and fat mass in females with excess fat (p=0.038). Plasma MCH concentrations rose significantly after feeding in a group of older individuals (n=50, males p=0.006, females p=0.023). There were no robust significant correlations between fasting or post-prandial MCH and resting metabolic rate, plasma glucose, insulin or leptin concentrations although there were correlations between circulating MCH and leptin concentrations in older individuals (p=0.029). These results indicate that the role of circulating MCH may not be reflective of its regulatory hypothalamic role.