To reduce N pollution by dairy cows, two levels of rumen degradable protein balance (RDPB) and the use of protected protein sources were studied in two feeding trials. Trial 1 investigated if the effect of lowering the RDPB, at a constant DPI (digestible protein in the intestine) supply, could be compensated by using protected protein sources (formaldehyde treated (= formolated) soybean meal (FSBM) or a combination of FSBM and formolated rapeseed meal at a ratio of 60:40). Trial 2 studied if FSBM is as effective as soybean meal (SBM) at a low RDPB level. Each trial was set up as a Latin square design, with three treatments and three periods, involving 18 lactating Holstein cows. Reducing RDPB from about 150 g/d to below -40 g/d resulted in small negative effects on the milk production, independent of the protein source (protected or not). However, N excretion per kg fat and protein corrected milk (FPCM) and N efficiency were both improved. At a low RDPB level, the performance of FSBM (35.0 kg FPCM) was no better than after feeding a combination of FSBM and formolated rapeseed meal (34.7 kg FPCM). The use of FSBM (32.8 kg FPCM and 34.2% N efficiency) was as effective as SBM (32.5 kg FPCM and 34.2% N efficiency). As such, SBM use can approximately be halved by using FSBM, possibly in combination with locally produced rapeseed meal.