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As part of an initial study we have attempted to ascertain whether lactating women exhibit high, clearly measurable levels of immunoreactive calcitonin (CT). Plasma levels of CT were measured in women who had been lactating for either a short time (less than 4 months) or a very long time (17-73 months). In 2 of 18 women lactating for only a short time CT levels were high and easily measured (greater than 200 pg/ml) while, in contrast, CT levels were undetectable (less than 60 pg/ml) in all of 16 nonlactating control subjects. In all six of the women studied who had been lactating for a long time, measurable levels of CT were observed, although provocative testing of women with measurable plasma CT using iv pentagastrin and calcium did not show a consistent pattern of CT secretory response. Thus, out of the total of 24 lactating women studied, one third were found to have measurable plasma levels of CT. The findings agree with studies in experimental animals which have indicated hypercalcitoninemia during lactation, but the factors responsible for the elevated CT levels during lactation remain to be elucidated.