The objective of this study was to investigate the fate of transgenic cry1Ab DNA and the encoded Cry1Ab protein during the metabolic degradation of dietary feed components in dairy cows and a potential transfer to blood, milk, feces or urine. A 25-month long-term feeding trial was conducted on thirty-six Simmentaler cows allocated in two groups fed diets containing either genetically modified corn (MON810, N=18) or the near-isogenic corn variety (N=18). The nutrients and energy contents of both maize varieties were comparable, ensuring equivalent feed conditions. Due to infertility or other production associated diseases, nine cows per group had to be culled and were replaced by heifers. Feed samples were collected weekly, whereas samples for feces, blood and milk were collected monthly, urine samples were taken bimonthly. All samples were analyzed for cry1Ab DNA by means of end-point PCR (feces, blood, urine) and quantitative real-time PCR (feed, milk). A sensitive and highly specific ELISA, optimized to quantify immunoreactive fragments of the Cry1Ab protein, was used to determine the recombinant protein in the collected samples. Non-transgenic feed samples were free of recombinant DNA and protein within the limit of detection, while in transgenic feed samples both, a 206 bp fragment of cry1Ab and immunoreactive fragments of the Cry1Ab protein were present. In contrast, all blood, milk and urine samples were free of recombinant DNA and protein. The cry1Ab gene was not detected in any fecal sample, whereas immunoreactive fragments of the Cry1Ab protein were detected in feces from all cows fed transgenic feed. Milk of dairy cows fed genetically modified corn for 25 months should be classified not different from milk of cows fed non-transgenic corn.