Prostaglandin synthase I (COX I) catalyzes the synthesis of prostaglandin H2 from arachidonic acid. The inhibition of prostaglandin synthase is critical for pain relief, with nonsteroidal anti-inflammatory agents (NSAIDs) commonly prescribed therapeutically. NSAIDs inhibit COX I by one of two distinct mechanisms: classical competitive (time-independent) inhibition or tight binding (time-dependent) inhibition. The structures of COX I crystallized in the presence of time-dependent and time-independent inhibitors were determined to be identical, suggesting that the variation in the inhibitory mechanism resulted from structural differences in the inhibitors themselves. To test this hypothesis, a series of NSAIDs were synthesized and the structural requirements for time-dependent inhibition assessed. Initial studies indicate that a phenyl-hydroxyl hydrogen bond between the inhibitor and enzyme is critical for time-dependent inhibition. New NSAIDs are being designed and synthesized to test this hypothesis.