In cultured porcine and human thyroid cells, electrophysiological and morphological studies showed that cultivation in the presence of TSH, prostaglandin E2 (PGE2) or dibutyryl cyclic AMP (dbcAMP) maintained normal cell polarity with iodine incorporation and organification. Cells cultivated in the absence of these substances had inverted cell polarity and lacked iodide incorporation. In the presence of TSH, PGE2 or dbcAMP, the thyroid cells formed follicles with normal cell polarity and the microvilli pointed toward the follicle lumina. Intracellular resting potentials were -60mV and the electrical potentials in the follicle lumina were negative at -20mV. The transmembrane potential differences (p.d.) between the follicle lumina and the epithelial cells were -40mV and those between the epithelial cells and the culture media -60mV. In the absence of TSH, PGE2 or dbcAMP, the thyroid cells formed 'domes' or hollow spheres with inverted cell polarity and the microvilli pointed toward the culture media. Intracellular resting potentials were -40mV, being less negative than those in the presence of TSH, PGE2 or dbcAMP. The electrical potentials in the 'dome' or hollow sphere cavities were positive at +19mV. The transmembrane p.d. between the culture media and the epithelial cells was -40mV and that between the epithelial cells and the cavities -60mV, indicating that electrophysiologically the cell polarity was inverted in the absence of TSH, PGE2 or dbcAMP. No significant differences in electrophysiology and iodine metabolism were observed between normal and Graves' human thyroid cells in culture.