We examined immunocytochemically the occurrence of the three peptides calcitonin, calcitonin gene-related peptide (CGRP), and gastrin-releasing peptide (GRP) in medullary carcinoma of the thyroid. We also sought to determine whether the plasma levels of these peptides were increased when stimulated with calcium and pentagastrin in familial medullary carcinoma of the thyroid (MCT). The tumor tissue from all 17 cases examined was found to exhibit calcitonin and CGRP immunoreactivity, and in 15 of the 17 cases the tumor tissue also contained GRP immunoreactivity. In 7 of the cases selected at random, an intravenous injection of calcium carbonate (2 mg/kg body weight) and pentagastrin (0.6 microgram/kg body weight) produced marked elevation in plasma levels of calcitonin but did not significantly alter the plasma levels of CGRP or GRP. We conclude that most MCT tumors contain CGRP and GRP immunoreactive cells but that the plasma levels of CGRP and GRP are not altered on stimulation. This finding is clearly in contrast to the markedly elevated calcitonin levels. Hence, determination of plasma calcitonin levels still seems to be the most appropriate diagnostic test for MCT.