Guinea pig thyroid tissue was incubated in a tissue culture system in the presence of various TSH concentrations and for various time periods. The morphokinetic response was examined with the electron microscope. The qualitatively finer aspects of the TSH-caused sequence of events are described and compared with in vivo results. After in vitro stimulation the general changes of cell membranes and of the cytocavitary network are very similar to those in vivo. On the other hand, there are differences with regard to intensity and duration of the structural response to TSH and additionally degenerative changes of organelles are found which might be explained by incubational factors. The role of the mitochondria as a source of energy is demonstrated by typical qualitative and quantitative alterations. An interesting finding is the degradation of mitochondria within colloid droplets, a mechanism which is suggested to represent a modified autophagy. Intracellular follicles are occasionally found during in vitro experiments. The formal genesis of this structure and its possible role is discussed. The present study indicates that this incubation model provides a useful tool for further analyses of thyroid disorders.