Iodide is shown to inhibit the cholinergic stimulation of prostaglandin E2 (PGE2) and F2 alpha (PGF2 alpha) synthesis in dog thyroid slices. The inibitory effect of iodide was already detectable at 1 micron and reached its maximal level at 10 microns. At this concentration, iodide inhibited the carbamylcholine-stimulated release of PGE2 and PGF2 alpha by 59% and 73%, respectively (eight experiments). The effect of iodide was neither immediate nor rapidly reversed after a change of incubation medium. The unstimulated release of PGE2 and PGF2 alpha and the stimulation by epinephrine and ionophore A23187 were not modified by iodide in concentrations up to 0.1 mM. The effect of iodide was suppressed in the presence of methimazole (0.1 mM) but not NaClO4 (2 mM). Iodid (0.1 mM) did not inhibit the stimulation by carbamylcholine of PGF2 alpha and PGE2 release by rat pancreas in vitro. These data demonstrate a new action of iodide on the thyroid and establish a link among iodide metabolism, PG synthesis, and cholinergic action in the dog thyroid.