The use of either purine bases (PB) in duodenal digesta or the urinary excretion of purine derivatives as an index of the ruminal microbial supply is based on the assumption that only a negligible amount of dietary nucleic acids (NA) reaches the duodenum. Such assumption is based on a rapid degradation of free NA incubated with rumen content and a constant RNA/DNA ratio in rumen digesta similar to values obtained in mixed rumen bacterial samples, indepent of feed ratio (McAllan & Smith, 1973). However it is generally accepted mat NA tend to overestimate rumen microbial synthesis due to dietary contamination of the microbial marker in the duodenum (Smith et al. 1978). This experiment aimed to determine the duodenal contribution of non-microbial NA by using 15N incorporation in PB, and the degradation rate of dietary PB by the “in situ” method.