Eighteen normal thyroid glands and unaffected thyroid tissue adjacent to 37 follicular cell-derived benign and malignant tumors and to ten thyroid metastases were studied immunocytochemically with calcitonin (CT) and prosomatostatin/somatostatin (SMS) antibodies. CT- and SMS-immunoreactive cells were found in 100% of cases, though with ample variations in number. Most but not all SMS-immunoreactive cells also contained CT. Diffuse and/or nodular C-cell hyperplasia was seen in 30% of pathological thyroid glands; in concomitance with follicular adenomas, the mean C-cell number more than doubled that found in normal glands. Furthermore the proportion of SMS-immunoreactive C-cells increased from about 1% of CT-immunoreactive cells in normal adult thyroid glands to 2.5% in follicular adenomas, 3% in follicular carcinomas, 4.6% in papillary carcinomas, and 5.7% in metastases. The findings suggest that C-cell hyperplasia may be causally related to pathologic disorders affecting follicular cells. Furthermore, the demonstration that the intrathyroidal SMS cell mass is readily affected by alterations of the follicular structure of the gland suggests a possible regulatory role of SMS in the thyroidal microenvironment.