Using a sensitive calcitonin (CT) immunoassay and a newly developed bioassay capable of detecting 0.025 MRC mU CT, we have studied acute and chronic plasma CT fluctuations in male and female rats. Immunoassay of serial plasma samples revealed progressive increases in plasma CT concentrations during aging; female rats have higher CT concentrations than age-matched males. Acute periodic CT fluctuations were discovered by immunoassay of plasmas obtained at 3-h intervals; the greatest values occurred just before and during feeding. Fed rats have higher CT than starved rats. We have used immunoadsorbent chromatography to concentrate specifically CT moieties from large volumes of plasma for concurrent immunoassay and bioassay measurements of circulating CT. These concurrent measurements of immunoextracted plasma CT demonstrate that for normal rats, our immunoassay measurements correspond to bioassay measurements. In 1-yr-old rats on a regulated feeding schedule, the biological (hypocalcemic) activity of CT recovered from 15 ml peripheral plasma ranged from less than 0.15 mU in starved males to 0.78 mU in feeding females. We conclude that biologically active CT circulates in normal rats and that the blood concentration of biologically active CT progressively increases during somatosexual maturation, being highest in old females, and increases acutely just before and during feeding.