We investigated whether inadequate dietary protein would result in increased serum parathyroid hormone (PTH) concentration, consistent with secondary hyperparathyroidism. Data from 2 controlled feeding studies were utilized. In study 1, 26 healthy women (15 young, 21-46 y, and 11 elderly, 70-81 y) consumed for 12 d each in separate trials 3 levels of protein, 1.00, 0.75, and 0.50 g protein/(kg. d). Blood was drawn from fasting subjects on d 12 of each trial. In study 2, 24 persons (54-80 y) were fed diets with either 1.20 g protein/(kg. d) for 2 wk (HPro, n = 11, 6 men, 5 women) or 1.2 g protein/(kg. d) for 1 wk and then 0.50 g protein/(kg. d) for a 2nd week (IPro, n = 13, 6 men, 7 women). Blood was obtained from fasting subjects after wk 1 and 2. Consistent with altered protein metabolism, urinary total nitrogen excretion and blood urea nitrogen fell progressively with decreasing protein intake in study 1; in study 2, the values decreased from wk 1 to 2 in the IPro group only. Serum intact PTH concentrations did not differ among the 3 protein intakes in study 1, or between the HPro and IPro groups in study 2. These findings do not support the hypothesis that the short-term ingestion of inadequate dietary protein increases serum PTH concentration.