Sensitive and selective high-performance liquid chromatographic assays for diclofenac and its monohydroxylated metabolites in biological fluids are described. Using ultraviolet detection at 282 nm, diclofenac is assayed in plasma at concentrations down to 10 ng/ml; total (free + conjugated) diclofenac and its monohydroxylated metabolites (the sum of 3'- + 4'-hydroxydiclofenac and 5-hydroxydiclofenac) are assayed in urine after chemical hydrolysis at concentrations down to 200 ng/ml. The applicability of the described assays is shown.