Three experiments were carried out on four rams fitted with rumen cannulas in a 4 x 4 Latin square design to investigate the effect of varying source and level of fat addition on microbial protein net synthesis (MN) in sheep fed meadow hay. The addition of rape seed oil (RSO) was tested in the 1^st experiment, the addition of linseed oil (LSO) in the 2^nd, and tallow (TAL) in the 3^rd experiment. Treatments in all experiments were similar: a control group without added fat and experimental groups with 4, 8 and 10% of fat in DM of diet. The fat source did not affect microbial protein net synthesis, but addition of 8% of fat tended to increase MN. The highest level of all purine derivatives was observed when RSO was added to meadow hay, whereas tallow addition decreased level of all purine derivatives. The effect of fat was well demonstrated in the variation in pH, and the levels of NH₃-N and volatile fatty acids.