The short-term effects of a simulated cattle dung pat on N-2 fixation and total uptake of N in a perennial ryegrass/white clover mixture was studied in a container experiment using sheep faeces mixed with water to a DM content of 13%. We used a new N-15 cross-labelling technique to determine the influence of dung-pat N on N-2 fixation in a grass/clover mixture and the uptake of dung N in grass and clover. The proportion of N in clover derived from N-2 fixation (%Ndfa) varied between 88-99% during the 16 weeks following application of the dung. There was no effect of dung on the %Ndfa in clover grown in mixture, whereas the %Ndfa in clover grown in pure stand decreased (nominal 2-3%) after dung application. Dung did not influence the amount of N-2 fixed, and the uptake of dung N in grass and clover proceeded at an almost constant rate. After 16 weeks, 10% of the applied dung N was taken up by grass and clover, 57% had been incorporated in the soil by faunal activity and 27% remained in residual dung on the soil surface. The dung N unaccounted for (7%) was probably lost by ammonia volatilisation and denitrification. The uptake of dung N in grass/clover mixtures in the field was similarly followed by using simulated N-15-labelled dung pats. The total dry matter production and N yields increased in the 0-30 cm distance from the edge of the dung patch, but the proportion of clover decreased. Thirteen months after application of the dung 4% of the applied dung N was recovered in the harvested herbage, 78% was recovered from the soil and the residual dung, and 18% was not accounted for. It is concluded that N-2 fixation in the dung patch border area in grass/clover mixtures is not influenced directly by the release of N from dung pats in the short term. However the amount of N-2 fixed may be reduced, if the growth of clover is reduced in the patch border area.