This study aimed to determine the relationship of some blood biochemicals, thyroid hormones profiles and minerals during different stages of the estrous cycle and in relation to follicular size with early conception in Egyptian buffaloes during the postpartum period. According to pregnancy diagnosis, 30 out of 43 buffaloes became pregnant (69.7%), 20 animals being
pregnant (46.6%) within 90 day-postpartum (1st group, G1) and 10 animals (23.25%) within 120 day postpartum (2nd group, G2). The estrous cycle was classified into four stages (I: 0-4d, II: 5-10d, III: 11-17d and IV: 18-21d). Follicles were grouped into three diameter classes: class 1 (3 to 5 mm), class 2 (6 to 9 mm), and class 3 (≥10 mm). Results showed that overall mean of serum total proteins (TP), albumin (AL), globulin (GL), glucose, cholesterol (CH) and triglycerides (TG) concentrations were higher (P<0.001) in G1 than in G2. Overall meanof TP and glucose levels increased (P<0.05) from I to II stage, then showed insignificant changes at the following stages, while GL and CH levels significantly (P<0.05) increased
from I up to III stage, then showed insignificant change, but AL and TG levels were not affected significantly by estrous stage. Overall mean of TP, AL, GL and TG levels were not affected significantly by follicular size. However, glucose concentration increased (P<0.05) by increasing follicular size from I to III class, while CH concentration increased (P<0.05)
only from I to II class. Concentrations of serum T3 and T4 as well as Zn and Se contents were higher (P<0.001) in G1 than in G2. As affected by follicular size, Se content increased (P<0.05) by increasing follicular diameter. However, concentration of T3and T4, and Zn contents were not affected by follicular size. The blood composition regarding biochemical, mineral and thyroid hormones provides a useful indication of the requirements for lactating buffaloes during early stage of postpartum
period. Thyroid hormones and biochemical metabolites concentration have relationships considerably with the stage of estrous cycle and follicular size. There were changes in the biochemical composition of blood happened during estrous stages, and follicular development, indicating a relationship of blood metabolites, mineral and thyroid hormone with the
reproductive status and metabolic activity of the lactating buffaloes. Based on the obtained results, breeders should take in mind the change in energy balance and feeding requirements during the early stage of postpartum period to obtain higher pregnancy rate within 90 day-postpartum