The aim of this study was to evaluate the effect of different nitrogen doses and five period of sample collection, on soil microbial biomass -nitrogen (SMB-N), total nitrogen (total N) and percentual ratio of the microbial biomass and total N (SMB-N/total N) in a Oxisol cultivated with barley (Hordeum vulgare L.).The experiment was installed in June, 2005, in an area located at Embrapa Cerrados, Federal District.The experimental design was a randomized block, with three replicates.The plots received doses of nitrogen: 20 -40 -80 kg ha -1 N and a control without it and the subplots were period of soil sample.Three applications of N were realized: 10 kg ha -1 on the 5 th day (06/14) after sowing; the rest of N was parceled in two applications with fertigation, on tillage, on the 27 th (07/08) DAP, e no 43 rd (07/22) DAP.Soil samples layer (0 -10 cm deep) were collected for (SMB-N) determination and total N in six periods: 02 days before of the first fertigation; 02 days after of the first fertigation; 04 days before of the last fertigation and 04 days after of the last fertigation; on flowering stage and after harvesting.There was effect of the doses of N and the period of soil collection on the SMB-N, total N and in the ratio SMB-N/ total N.The average values of total N revealed steadier in short-term (cycle of the culture) and this was not a good parameter to evaluate the behavior and N transformations in the soil-plant system.