The present study was aimed to test the possible cyclooxygenase (COX)-1/COX-2 selectivity of the old analgesic drug phenacetin and its metabolite p-phenetidine, which exhibits high renal toxicity. Paracetamol (acetaminophen), the main metabolite of phenacetin with low renal toxicity, and indomethacin were selected as reference compounds. Collagen-stimulated platelet thromboxane B2 (TxB2) production and phorbol 12-myristate-13-acetate (PMA)-induced neutrophil prostaglandin E2 (PGE2) synthesis were used as indicators for COX-1 and COX-2 activity, respectively. Phenacetin was even less potent than paracetamol to reduce the production of both TxB2 and PGE2, and no clear preference for either of the COX-enzymes was seen. P-phenetidine was a more potent inhibitor, already at nanomolar level, of the synthesis of these prostanoids than indomethacin and showed some preference to COX-2 inhibition. Somewhat higher, micromolar, concentrations of p-phenetidine also reduced COX-2 expression in neutrophils. We suggest that the very potent inhibitory activity of p-phenetidine on PGE2 synthesis combined with the reduction of COX-2 expression could explain the renal papillary necrosis in phenacetin kidney.