Upon treatment with TSH the intracellular location of 3’, 5’-nucleotide phospho diesterase changes with time. Before hormonal treatment the histochemical reaction detects the enzyme at the periphery of the thyroidocyte. One hour after TSH administration the enzyme seems concentrated at the apical part of the cells. Two hours after hormonal treatment the stain appears scattered throughout the cytoplasm. The coincidence of enzyme location with the possible cyclic AMP sites of action is stressed.