We examined the effects of thyrotropin (TSH), prostaglandins E1 and E2 (PGE1, PGE2), and of the prostaglandin antagonists 7-oxa-13-prostynoic acid (PY1) and 7-oxa-l5- hydroxy-13-prostynoic acid (PY2) on adenyl cyclase activation and cyclic AMP formation in isolated bovine thyroid cells and canine thyroid slices, respectively. In the presence of ineffective or minimally effective concentrations of one stimulator (TSH or PGE), adenyl cyclase activation and cyclic AMP formation induced by a maximally effective concentration of the other stimulator were markedly reduced. Combinations of maximally effective concentrations of TSH and PGE had no additive effect on adenyl cyclase activation or cyclic AMP formation. PY1 and PY2 inhibited PGE1} PGE2, and TSH stimulation of adenyl cyclase in isolated bovine thyroid cells. PY1 blocked both PGE2 and TSH effects on cyclic AMP formation in canine thyroid slices. PY1 blocked TSH and PGE1 effects on phagocytosis of latex beads by isolated bovine thyroid cells as well as TSH and PGE2 effects on colloid droplet formation in canine thyroid slices, but in neither instance modified dibutyryl cyclic AMP (DBcAMP) effects thereon. These findings suggest that thyroidal prostaglandin may play an important role in TSH stimulation of cyclic AMP formation and hormonogenesis in the gland. (Endocrinology90: 343, 1972)