Abstract: The aim of this study was to investigate the way Pea Seeds (PS) can affect some blood metabolites and rumen parameters in lambs being fattened through consumption of sainfoin (Onobrychis viciifolia) hay (SH). The main nutrient source in the groups was SH. The SH was partially replaced by PS in two of three groups of animals. About 24 16 week-old Norduz female lambs (31.72±2.47 kg) were divided into three groups and fed their diets. SH and PS were available in the diets in the following SH:PS ratios (on a body weight basis): 3:0 in the first (control) group (C), 2:1 in the second group (PI) and 1:2 in the third group. The diets of the groups were of different nutritional content. The animals were split randomly between the three groups (C, PI and PII). The research was completed in 25 days and the animals were fed twice a day over this period. Blood and rumen liquid samples were taken twice: one pre-trial (B) and the other post-trial (A). Blood Triglyceride (TG), Cholesterol (CH), Creatinine (CR), Direct Bilirubin (DB) and potassium values and increases in Live Weight (LW) of the animals were not found to be significant due to the dietary treatment. However, Protozoa Numbers (PN), Total blood Protein (TP), Albumin (AL), Urea (UR), Globulin (GL), Chlorine (Cl) and sodium (N) values and the rumen NH level increased significantly by the end of the trial (p<0.05).