The non-steroidal anti-inflammatory drugs diclofenac (DCF) and mefenamic acid (MFA) were treated with the white rot fungus Phanerochaete sordida YK-624. DCF completely disappeared and MFA decreased by about 90% after 6 days of treatment. It was also confirmed that the fungus almost completely removed the acute lethal toxicity of DCF and MFA towards the freshwater crustacean Thamnocephalus platyurus after 6 days of treatment. Mass spectrometric and (1)H nuclear magnetic resonance analyses demonstrated that two mono-hydroxylated DCFs (4'-hydroxydiclofenac and 5-hydroxydiclofenac) and one di-hydroxylated DCF (4',5-dihydroxydiclofenac) were formed via fungal transformation. The four metabolites of MFA were identified as 3'-hydroxymethylmefenamic acid (mono-hydroxylated MFA), 3'-hydroxymethyl-5-hydroxymefenamic acid (di-hydroxylated MFA), 3'-hydroxymethyl-6'-hydroxymefenamic acid (di-hydroxylated MFA) and 3'-carboxymefenamic acid. These results suggest that hydroxylation catalyzed by cytochrome P450 (CYP) in P. sordida YK-624 may be involved in the elimination and detoxification of DCF and MFA. This notion was further supported by the fact that smaller decreases in DCF and MFA were observed in cultures of P. sordida YK-624 incubated with 1-aminobenzotriazole, a known inhibitor of CYP.