Abstract. Pheochromocytomas and adrenal medullary hyperplasia were investigated ultrastructurally and by assaying catecholamines in tissue and urine. Three of seven bulls had concomitant thyroid C-cell neoplasms and normal parathyroid glands. Pheochromocytomas were either bilateral or unilateral, and were composed of large chromaffi cells predominantly in the storage phase of the secretory cycle. Two pheochromocytomas were composed of cells with ultrastructural characteristics of the epinephrine-secreting type and contained round secretory granules of low electron density. The norepinephrine-secreting type of chromaffi cell predominated in one pheochromocytoma and had storage granules with an electrondense, eccentric core and a wide submembranous space. Two pheochromocytomas were composed of a mixture of epinephrine-and norepinephrine-secreting cells. Adrenal medullary hyperplasia in four bulls consisted of multinodular or diffuse areas of hyperchromatic chromaffi cells that were non-encapsulated, and compressed adjacent cortical tissue. Tissue and urinary concentrations of norepinephrine were increased in bulls with adrenal medullary hyperplasia or pheochromocytoma, but urinary vanillylmandelic acid and unconjugated epinephrine levels were not different from those of control bulls with a normal adrenal medulla.An extensive survey of cattle at an abattoir showed that tumors of the adrenal gland usually occur in older cattle, and that pheochromocytomas are the second most common type of adrenal neoplasm [23]. A 6% incidence of pheochromocytomas was reported in 402 bulls, with no breed predilection [22] but with a significant coincidental occurrence of ultimobranchial (C-cell) tumors in the thyroid gland [ 1 11. It has been suggested that the simultaneous occurrence of medullary thyroid carcinoma, pheochromocytoma, and parathyroid hyperplasia or adenoma in man represents a multicentric neoplastic transformation of cells of neural crest origin [4, 12,15, 161. During the development of certain familial syndromes of multiple endocrine neoplasms in man, the adrenal medulla undergoes nodular or diffuse hyperplasia before pheochromocytoma develops [3,7]. There have been few reports correlating ultrastructural findings with catecholamine assay of the proliferative lesions in the adrenal medulla that often develop in aged bulls. The objective of our study was to determine 316