Study on environmental factors influencing the urea content of cow's milk in Wallonia and estimation of nitrogen rejection. In Wallonia (South of Belgium), the urea content of milk produced on farms is assessed by the "Comité du lait". A total of 8,295,337 pieces of data recording the urea content in milk tanks was collected on Walloon farms in 2000 and during the period 2002-2011. These data were analyzed using a linear model. The fixed effects of the agricultural area or vulnerable zone, the month, the year and the interactions between these parameters were included in the model. The levels of fat and protein content in the milk were used as covariates. The average urea content in milk was found to be 255 mg·l-1. Four percent of the observations recorded a urea content of over 350 mg·l-1 and 14% recorded a urea content of over 400 mg·l-1. The models explained 38% and 35% of the variation in milk urea for the agricultural area and the vulnerable area variables, respectively. Within these models, the variables of month, region, year and the month-year interaction were the components that provided the most information. Milk urea content was higher in the summer period – April to October – than during the winter period – November to March – (287 vs 210 mg·l-1). This observation can be explained by the change in the cows' diet. During the summer, most of the cows grazed; ingestion of grass can lead to a nitrogen surplus in the diet. In the so-called "Limoneuse" and "Sablo-limoneuse" agricultural areas, recorded urea content was lower than in "Ardenne" and "Haute-Ardenne" (223 and 220 mg vs 278 and 284 mg·l-1, respectively). This difference can be explained by a greater use of grass in the diet in Ardenne and Haute-Ardenne. In vulnerable areas, urea content was lower (236 vs 273 mg·l-1). Annual nitrogen production calculated using different prediction equations ranged from 82 to 119 kg N per cow depending on the agricultural area.