The stable carbon and nitrogen isotopic composition of urine and milk samples from cattle under different feeding regimes were analysed over a period of six months. The isotope ratios were measured with isotope ratio mass spectrometry (IRMS). The delta13C values of milk and urine were dependent on different feeding regimes based on C3 or C4 plants. The delta13C values are more negative under grass feeding than under maize feeding. The delta 13C values of milk are more negative compared to urine and independent of the feeding regime. Under grass feeding the analysed milk and urine samples are enriched in 13C relative to the feed, whereas under maize feeding the 13C/12C ratio of urine is in the same range and milk is depleted in 13C relative to the diet. The difference between the 15N/14N ratios for the two feeding regimes is less pronounced than the 13C/12C ratios. The delta 15N values in urine require more time to reach the new equilibrium, whereas the milk samples show no significant differences between the two feeding regimes.