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A study of six months duration was conducted in lactating dairy cows by feeding them on paddy straw plus non-forage sources based complete rations containing different levels of neutral detergent fibre (NDF), in two phases, viz., early and mid lactation, to assess the nitrogen (N) balance. Three isonitrogenous and isocaloric complete rations, T1, T2 and T3 with 25, 30 and 35 per cent NDF, respectively, formulated as per ICAR (1998) were fed to the experimental cows. The average daily nitrogen (N) intake of cows fed on the three experimental rations was significantly higher (P<0.05) in T2 and T3 than T1, while T2 and T3 were similar (P>0.05). The average daily total N excretion, the N balance and per cent retention of N of animals fed on the three experimental rations, T1, T2 and T3 were similar (P>0.05). Cows in T1 which had significantly lower (P<0.05) N intake, excreted more N through the dung, urine and milk, with the result that they had a lower, but non-significant (P>0.05), N balance as well as per cent retention of N than those fed on rations T2 and T3. A thorough evaluation of the results obtained in the present study in phases I and II, showed that the positive N balance observed in cows fed on all the three experimental rations, indicated retention of protein, which in turn provided conditions that prevented performance loss, thereby ensuring that the requirements of protein in the diets were met. These results suggest that complete rations with 25 to 35 per cent NDF, containing paddy straw as the sole source of roughage NDF, with the rest of NDF being met from non-forage sources, can be recommended for use in lactating dairy cows, with 30 and 35 per cent being the most ideal NDF level. [ABSTRACT FROM AUTHOR]