Resorcylic acid lactones (RALs) are fungal metabolites with known biological activity. Zeranol, a synthetic RAL, has been used as an estrogenic growth promoter in cattle; however, its use is prohibited in several countries. Zearalenone, a mycotoxin produced by Fusarium spp., is commonly found in contaminated animal feed and can be metabolized into other RALs, which are subsequently excreted in urine. To differentiate between natural contamination from feed and the illegal administration of zeranol, the European Union Reference Laboratory for Growth Promoters (EURL) developed a mathematical equation. This study aims to evaluate the detection and quantification of RALs in bovine urine from animals fed zearalenone-contaminated diets, implanted with zeranol, or subjected to both conditions. RALs were detected and quantified in the urine of cattle consuming contaminated corn, while zeranol and taleranol were identified in the urine of implanted animals. The EURL equation proved to be a valuable tool for determining the origin of RALs in bovine urine and holds significant potential for monitoring and enforcing regulations regarding the illegal use of zeranol.