Both calcitonin and gastrin have been found in the mammalian central nervous system, including the pituitary. Following a pentagastrin infusion in several patients with medullary carcinoma of the thyroid, we noted a coincident increase in plasma calcitonin and prolactin (PRL) levels. In order to evaluate further the influence of pentagastrin on human PRL release, a pentagastrin infusion was administered to 13 patients with active medullary carcinoma of the thyroid (MTC), eight subjects with inactive MTC, eight family members without MTC, and ten normal subjects. Plasma mean +/- SE PRL levels were significantly (P less than 0.01) increased in the active MTC patients from 7.6 +/- 0.5 to 12 +/- 1.4 ng/mL by 15 minutes post pentagastrin. Plasma mean +/- SE calcitonin levels increased in parallel with the plasma PRL levels from 0.28 +/- 0.1 to a peak of 1.9 +/- 0.9 ng/mL at 5 minutes post pentagastrin. A significant (P less than 0.05) correlation was found between the percentage increase in plasma calcitonin concentrations and plasma PRL levels at five and ten minutes post pentagastrin stimulation in this group of active MTC patients. Significant increases in serum calcitonin levels in the other groups post pentagastrin were of lesser magnitude and were not associated with a significant increase in PRL release. This latter observation suggested that neither the stress of the infusion nor the multiple endocrine neoplasia type 2 nor the pentagastrin was responsible for the observed increase in plasma PRL levels in the active MTC patients. These findings suggest, but do not prove, that calcitonin is a PRL-releasing factor in humans.