Thyroid glands from 7 marmosets (Callithrix jacchus) of different age groups (newborn, 2 weeks, 2, 7, 8 and 11 months, and 8 years) were grown as organ culture according to Trowell at the medium/air interphase for 3 to 30 days. The morphology of thyroid tissue was well preserved until the end of the culture period. Necroses were only occasionally seen in connective tissue. In contrast to the in vivo situation, the number of lysosome-like inclusions changed considerably and cell-cell contacts became loose. After the addition of TSH the number of apical microvilli, apical granules and coated vesicles rose. The basal surface of follicle cells enlarged due to the formation of processes or ridges and invaginations. Mitosis could be observed. These findings show that, using the Trowell culture technique, thyroid tissue can be grown for up to 4 weeks without showing distinct changes in its morphology. Its responsiveness to the application of TSH is maintained in vitro. Hence, this technique is suitable also for long-term endocrinological, pharmacological and toxicological investigations.