Twenty‑four Friesian spring calving cows where divided into three groups of 8 cows, as a Latin square 3x3: 3 concentrates of different contents in non‑degradable protein, called fish concentrate (CP:48.6%); soya (CS:37.7%) and cotton (CA:42.5%), during three experimental periods of 35 days each and seven for adaptation, to study the effects on the production and chemical composition of the milk, serum and ruminal liquid in grazing conditions. Each of the sources of protein was included in 4.5 kg of dry matter per cow and day. The results did not show any difference in the milk production, but difference was a (P < 0.05) in fat corrected milk4; the concentrations of crude protein, casein and proteic nitrogen were not different between sources of protein, except (P <  0.001) for the last, between periods. The content of urea in milk was similar between CP and CS (329 mg/l) and different (P <  0.01 ) with respect to CA. The concentrations of serum have not very important differences, in urea values (mg/dl), glucose (mg/dl) and crude protein (g/dl), 24.1, 58.1 and 6.82; 26, 59.2 and 6.85; 29.9, 60.3 and 6.81 for the treatments CP, CS and CA respectively. The pH and N-NH₃, of ruminal liquid had not important differences between treatments.