A study of six months duration was conducted to evaluate the performance of lactating dairy cows fed on paddy straw plus non-forage fibre sources such as coconut cake and deoiled rice bran, based complete feeds with varying levels of neutral detergent fibre (NDF) on the basis of milk urea nitrogen (MUN), rumen ammonia and nitrogen (N) balance, in two phases of three months (90 days) duration each; viz., early and mid-lactation, respectively. Three isonitrogenous and isocaloric complete rations, T1, T2 and T3 with 25, 30 and 35 per cent NDF, respectively formulated as per ICAR-NIANP (2013) were fed to 18 lactating dairy cows, divided into three groups of six each. The daily milk yield was recorded; the fortnightly milk urea nitrogen, the nitrogen balance at the end of each phases and the rumen ammonia at the end of the total experiment, were estimated. The average daily dry matter intake of cows fed on the three experimental rations were similar (P<0.05), from the third fortnight onwards till the end of the experiment, even though the DMI tended to be higher in T3 and T2 as compared to T1. The milk yield of cows in all the dietary treatments were similar (P>0.05) throughout the experiment, even though it tended to increase with increase in NDF content of the ration. The milk urea nitrogen (MUN) content of cows decreased linearly and significantly (P<0.05), with increase in the NDF content. The N balance of cows fed on the experimental rations, T1, T2 and T3 in phases I and II were similar (P>0.05). The rumen ammonia production was significantly lower (P<0.05) in T3 than T1 and T2, with T1 and T2 being similar (P>0.05). These results reveal that cows in all the three dietary treatments performed well, with the complete rations T3 and T2, showing better performance than T1, suggesting that paddy straw plus non-forage sources based complete rations, having a total NDF of 25 to 35 per cent NDF can be recommended for use among early and mid-lactation dairy cows, with 35 per cent being the ideal NDF level, followed by 30 per cent.