The activity of acid phosphatase in rat thyroid follicle cells was studied cytochemically—both on the light and electron microscopic level—as well as biochemically. In glands suppressed by the administration of thyroxine, the enzyme was located in small bodies corresponding to the cytosomes (“lysosomes”). Five minutes after the administration of thyrotropic hormone (TSH), colloid droplets began to appear in the apical cytoplasm. Sixty to 75 minutes after the TSH administration most colloid droplets displayed acid phosphatase activity. It was found by quantitative cytochemical and biochemical measurements that, during the stimulation period, the number of acid phosphatase positive bodies decreased significantly, while the total enzyme activity remained constant. The findings were interpreted as indicating that a fusion between preexisting enzyme-containing bodies and newly formed colloid droplets had taken place. This interpretation was supported by the demonstration, in the electron microscope of images suggesting fusion between cytosomes and colloid droplets. The findings were correlated with cytochemical and biochemical data of normal thyroid tissue, and the physiological significance of the observations was discussed.