Remifentanil (4-methoxycarbonyl-4-[(1-oxopropyl)phenylamino]-1-piperidinepropionic acid methyl ester) is a mu-opioid receptor agonist with considerable abuse potential in racing horses. The identification of its major equine urinary metabolite, 4-methoxycarbonyl-4-[(1-oxopropyl)phenylamino]-1-piperidinepropionic+ ++ acid, an ester hydrolysis product of remifentanil is reported. Administration of remifentanil HCl (5 mg, intravenous) produced clear-cut locomotor responses, establishing the clinical efficacy of this dose. ELISA analysis of postadministration urine samples readily detected fentanyl equivalents in these samples. Mass spectrometric analysis, using solid-phase extraction and trimethylsilyl (TMS) derivatization, showed the urine samples contained parent remifentanil in low concentrations, peaking at 1 h. More significantly, a major peak was identified as representing 4-methoxycarbonyl-4-[(1-oxopropyl)phenylamino]-1-piperidinepropionic+ ++ acid, arising from ester hydrolysis of remifentanil. This metabolite reached its maximal urinary concentrations at 1 h and was present at up to 10-fold greater concentrations than parent remifentanil. Base hydrolysis of remifentanil yielded a carboxylic acid with the same mass spectral characteristics as those of the equine metabolite. In summary, these data indicate that remifentanil administration results in the appearance of readily detectable amounts of 4-methoxycarbonyl-4-[(1-oxopropyl)phenylamino]-1-piperidinepropionic+ ++ acid in urine. On this basis, screening and confirmation tests for this equine urinary metabolite should be optimized for forensic control of remifentanil.