Four slow releasing non-protein nitrogen (NPN) sources viz. urea-straw (US), urea-starch (UST), biuret-straw (BS) and biuret-starch (BST) were compared with cotton seed meal (CSM) as control nitrogen source. In all experiments, 2% nitrogen level on DM basis in contents of each fermentation flask were maintained through replacing 30, 50 and 70% of CSM by respective NPN source. Four in vitro experiments were `conducted to see the effects of various NPN sources on dry matter digestibility (DMD), microbial growth, ammonia nitrogen release and pH changes. The effects of NPN sources CSM, BST, UST, BS and US on DMD were significant (P<0.001) and the mean DMD values for the five NPN sources were 13.65, 31.76, 28.81, 27.14 and 22.39%, respectively. The UST resulted in maximum bacterial count of 290.0 X 107. The bacterial counts with US, CSM, BST and BS were significantly lower (P< 0.001) than that of UST. The NH3-N released due to NPN sources, levels, time of incubation and interactions between sources x levels, sources x time and levels x time were significant (P<0.001). The release of NH3-N by BST was maximum (17.77 mg/dl), followed by UST (17.35 mg/dl), BS (15.85 mg/dl) and US (15.67 mg/dl). The effect of various NPN sources on pH were also significant (P<0.01). The mean pH values were 7.13, 7.15, 7.08, 7.13 and 7.15 for US, BS, UST, BUST and CSM, respectively. The UST containing cultures had lowest pH values compared to other NPN sources.