It is difficult to accurately evaluate the loss of nitrogen in the environment following the spreading of pig slurry, because other nitrogen sources (fertilizers, crop residues, etc.) also contribute to the overall nitrogen losses. The radioactive labelling, with nitrogen 15, of the slurry nitrogen allows to distinguish it from the nitrogen found in other sources in the field. Radioactive labelling was used in a two-year assay (2003 and 2004) for determining the real fertilizing efficiency of pig slurry applied in post-emergence on field corn (at a rate of 85 to 95kg total N/ha) and for determining the real proportion of nitrogen loss attributable to slurry spreading. This study allowed us to establish the distribution of the slurry nitrogen in the soil, plants and environment. As measured at the end of the growing season, 20 to 50% of the slurry nitrogen was assimilated by corn, depending on the year and the type of soil of the assay. Nitrogen uptake was lower in a clay-rich soil, since clay reduces the availability of ammoniacal nitrogen. Fifteen to 25% of the slurry nitrogen was lost in the environment between spreading and harvest. The measured nitrogen uptake and losses of slurry are similar to those reported for mineral fertilizer applications at post-emergence of corn and at similar rates. In the various conditions of this study, spreading of pig slurry proved as efficient as the application of mineral fertilizer in terms of agronomic performances of corn, and no more at risk, environmentally speaking.