The cyclooxygenase activity of the membrane protein prostaglandin H₂ synthase isoform 1 (PGHS-1) is the target of the nonsteroidal antiinflammatory drugs (NSAIDs). The X-ray crystal structures of PGHS-1 in complex with the NSAIDs flurbiprofen and bromoaspirin have been determined previously [Picot, D., et al. (1994) Nature 367, 243−249; Loll, P. J., et al. (1995) Nat. Struct. Biol. 2, 637−643]. We report here the preparation and characterization of novel potent iodinated analogs of the NSAIDs indomethacin and suprofen, as well as the refined X-ray crystal structures of their complexes with PGHS-1. The PGHS−iodosuprofen complex structure has been refined at 3.5 Å to an R-value of 0.189 and shows the suprofen analog to share a common mode of binding with flurbiprofen. The PGHS−iodoindomethacin complex structure has been refined at 4.5 Å to an R-value of 0.254. The low resolution of the iodoindomethacin complex structure precludes detailed modeling of drug−enzyme interactions, but the electron-dense iodine atom of the inhibitor has been unambiguously located, allowing for the placement and approximate orientation of the inhibitor in the enzyme's active site. We have modeled two equally likely binding modes for iodoindomethacin, corresponding to the two principal conformers of the inhibitor. Like flurbiprofen, iodosuprofen and iodoindomethacin bind at the end of the long channel which leads into the enzyme active site. Binding at this site presumably blocks access of substrate to Tyr-385, a residue essential for catalysis. No evidence is seen for significant protein conformational differences between the iodoindomethacin and iodosuprofen of flurbiprofen complex structures.