Recent results of biomedical studies suggest that rumenic acid (RA), the major isomer of conjugated octadecadienoic acids (CLA), appears to have beneficial health effects in humans. The major source of RA in the human diet is milk and beef fat, but average intake is too low to exhibit a health-protective effect. In light of current studies, the total amount of RA available to humans also depends on the endogenous synthesis of RA through Δ⁹-desaturase activity with trans-vaccenic acid (VA) as the substrate. The results of experiments suggest that the endogenous synthesis of RA has positive effects on human health. The enrichment in VA and RA of bovine fat through the diet is well documented in the literature. Current research has demonstrated that Δ⁹- desaturase is responsible for more than 80% of RA in milk and beef fat and that the enzyme activity is affected by non-dietary factors. This review presents the current state of knowledge about the influences of breeds, stage of lactation, type of tissues, enzyme gene polymorphisms and interactions with other genes, nutrients and hormones at tissue level on the endogenous synthesis of RA in cattle.