The synthesis of a series of 2-anilinophenylacetic acids, close analogues of diclofenac, is described. These compounds were tested in two models used for evaluating the activity of nonsteroidal antiinflammatory drugs (NSAID's), inhibition of cyclooxygenase enzyme activity in vitro, and adjuvant-induced arthritis (AdA) in rats. Statistically significant correlations were found between the inhibitory activities of the compounds in these two models, indicating that cyclooxygenase inhibition seems to be the underlying mechanism for the antiinflammatory activity of these compounds. Quantitative structure-activity relationship (QSAR) analysis revealed that the crucial parameters for activity in both models were the lipophilicity and the angle of twist between the two phenyl rings. Optimal activities were associated with halogen or alkyl substituents in both ortho positions of the anilino ring. Compounds with OH groups in addition to two ortho substituents or compounds with only one or no ortho substituents were less active.