The majority of Graves' immunoglobulins have the capacity to stimulate thyroid adenylate cyclase (AC) activity in vitro. So far, the exact events leading to AC activity stimulation by thyroid stimulating antibody (TSAb) are not known. It has been suggested that TSAb activates thyroid AC through prostaglandin (PG) synthesis, implying the concept that TSH and TSAb stimulate AC in different ways. If this is so, the inhibition of PG synthesis should notably reduce the response of thyroid plasma membrane to TSAb. We have, therefore, investigated the effect of 2 inhibitors of PG synthesis on the TSAb-stimulated AC activity in a crude human thyroid plasma membrane preparation. Neither indomethacin, nor hydrocortisone, even at concentrations able to completely inhibit PG synthesis, had any significant effect on the thyroid plasma membrane response to TSAb. The results suggest, therefore, that, at least under our experimental conditions, PG does not mediate the AC activation by TSAb. Consequently, the aforementioned suggestion should not be used to claim that TSH and TSAb activate thyroid AC through different pathways. The data, however, do not exclude that TSAb may determine PG synthesis in thyroid cells through the phosphatidylinositol signal system (Pl system).