Hyperactive dog thyroids were prepared by repeated TSH stimulation in vivo. Butanol-extractable 125I (BE125I) release in vitro from slices and hormonal secretion in vivo in the thyroid vein was enhanced. Apical pseudopods and colloid droplets were very infrequent in such hyperactive thyroids. Moreover, BE125I release was insensitive to cytochalasin B, to inhibitors of microtubules, and to metabolic inhibitors, and decreased only with temperature. Our data demonstrate that hormonal secretion by hyperactive thyroids is not secondary to apical phagocytosis (i.e. macropinocytosis). Micropinocytosis or intraluminal hydrolysis of thyroglobulin are suggested as first step of the secretory process.