To study the production of prostacyclin (PGI2) and thromboxane A2 (TxA2) in endometriosis in vitro, samples of endometriotic tissue taken during operation from 6 women were superfused for 4.5 hours in 95% O2/5% CO2 at 37 degrees C, and the stable metabolites of PGI2 (=6-keto-PGF1 alpha), and TxA2 (=TxB2) were measured by radioimmunoassays from the superfusates. All samples studied produced 6-keto-PGF1 alpha in the range from 0.2 to 10.5 nanograms/gram of dry tissue/minute with a mean of 3.6 ng/g/min during the whole experiment. TxB2 was also released by each sample at rates between 0.2 and 11.9 ng/g/min (mean 2.6 ng/g/min). The production of these prostanoids tended to be greater in the serosal (n = 2) than in the ovarian (n = 4) endometriosis. The addition of indomethacin of 10(-5) - 10(-3) moles/l to the superfusion medium inhibited concentration-dependently the synthesis of these prostanoids. Apart from these in vitro data implying the production of PGs in endometriosis, 18 patients with pelvic endometriosis sustained no relief for their endometriotic symptoms from the treatments with three anti-prostaglandins (acetylsalicylic acid, indomethacin, tolfenamic acid) in a double-blind, placebo-controlled trial.