Subconfluent FRTL cells were examined in the electron microscope after the following treatments: i) continuous TSH stimulation; ii) starvation from TSH for 3 or 7 days; iii) seven days of TSH deprivation followed by two days of TSH stimulation. The organelle complement of cells grown in the presence of TSH appeared to be consistent with their secretory properties. Rough endoplasmic reticulum (RER), in the form of round vesicles, and Golgi apparatus, were quite prominent. Cells were not properly polarized. Their dorsal surface was covered with microvilli and occasional pseudopods. After TSH withdrawal the cells flattened on the dish, lost most of their plasma membrane specializations and reorganized actin stress fibers. RER shape was modified from round vesicles to flat cisternae which thereafter almost completely disappeared. The Golgi apparatus did not seem to be modified. Autophagic vacuoles became more prominent. All the modifications were fully reversed after TSH replacement indicating an hormonal regulation in the amount and morphology of some organelle, in particular of the RER.