Six groups of thyroid glands ofScarus dubius were examined and compared by electron microscopy after anin vitro culture for 4h with graded doses of bovine thyrotropin (bTSH). Five doses of bTSH were used encompassing the full range of the dose-response curve developed for this tissue. Upon electron microscopic examination, micrographs were taken randomly and at the same magnification, and three intracellular inclusions were quantified. The relative surface density of rough endoplasmic reticulum (rER) and the relative surface area of lysosomes and engulfed colloid droplets were recorded for each group. Three treatment groups, (1) control, no bTSH, (2) tissues exposed to 1 mIU/ml bTSH, and (3) tissues exposed to 2 mIU/ml bTSH, did not differ from each other in the quantified organelles nor in general appearance. Overall, these three groups were similar in appearance to the ultrastructure described in other teleosts except for a lack of flagellated cells. Compared to the first three groups, treatment with 5 mIU/ml bTSH, increased the density of rER, and the proportion of cell area occupied by lysosomes and engulfed colloid. This group also possessed either more microvilli or pseudopods at the lumenal surface of the follicular epithelium. After exposure to 10 mIU/ml bTSH there was an even greater increase in surface density of rER, and in surface area occupied by lysosomes and engulfed colloid droplets. The apical portion of this group was highly irregular, commonly displaying pseudopods. Group (6), (20 mIU/ml), showed a decline in cytoplasm in comparison to group (5) with many epithelial cells breaking apart. A few cells in this group were still intact but contained huge engulfed colloid droplets which extended from the basal to apical borders. This first detailed description suggests that the teleost thyroid gland undergoes ultrastructural changes with exogenous TSH stimulation in a manner similar to that seen in higher vertebrates.