Decrease in connexin (Cx) expression and loss of gap junctional intercellular communication (GJIC) have been associated with aberrant cell growth and enhanced neoplastic phenotype. We studied the expression of Cx26, Cx32, Cx43, and Cx50 in chromaffin cells of 10 normal human adrenal glands, 10 benign, and 13 malignant pheochromocytomas. Immunohistochemistry showed that Cx50 expression seemed to be the predominant form of Cx expressed in human chromaffin cells, whereas Cx43 immunoreactivity was the most prominent form found in the adrenal cortex. However, Western blotting revealed that nonphosphorylated and singly and doubly phosphorylated forms of Cx43 were present within the malignant adrenomedullary cells at the same time. Cx26 and Cx32 were distributed inhomogeneously with no emphasis of expression in the types of tissues studied. Cx50 expression was diminished in malignant pheochromocytomas. We conclude that there is a differential expression of Cx in human adrenals. Immunohistological testing for Cx expression does not however allow differentiation of benign from malignant pheochromocytomas.