Technical Abstract: Molybdenum is an essential nutrient in the human diet and is a cofactor for several enzymes. While intakes can vary widely with geographical region, both deficiency and toxicity in humans are rare. To determine if humans have adaptive mechanisms for maintaining molybdenum status under varying intake levels, two clinical studies were performed. In the first study, healthy males consumed a very low molybdenum diet for 102 days, followed by a high molybdenum diet for 18 days. In the second study, healthy males were administered dietary molybdenum at five different levels consecutively, consuming each intake level for 24 days. In both studies, tracer doses of molybdenum were given orally and intravenously at several time points during the study, and plasma, urine, and fecal samples were analyzed for tracer and tracee molybdenum. Dynamic compartmental modeling of tracer and tracee data was performed using the WinSAAM software package. Modeling results revealed that two physiologic pathways were sensitive to molybdenum exposure: increasing molybdenum intake resulted in (1) increased urinary molybdenum excretion and (2) decreased fraction of molybdenum deposited into tissues. The ability for these pathways to adjust to changes in molybdenum intake may explain why molybdenum deficiency and toxicity in humans is rare.