The ruminal microbial protein synthesis (MPS) was estimated in six adult Granadina goats and six Segureña sheep by measuring the urinary allantoin excretion and using the purine bases/N (PB/N) content ratios, determined in liquid associated bacteria (LAB) from continuous fermentors. Both, animals and fermentors were fed four different diets which included olive by-products: twostage olive cake and olive leaves. Goats showed higher (P<0.001) microbial protein synthesis and efficiencies values for some diets than sheep. The PB/N content was affected (P<0.001) by the quality of diet supplied to the fermentors and its use resulted in an overestimation of MPS.