The study was carried out on 16 clinically normal stallions of Polish Primitive Horses in the in the reproductive season (April - July). Four stallions were infused with endotoxin (LPS) from Escherichia coli 055:B5 at doses of 0.3 mg/kg b.w. (group ENDO). Twelve stallions were treated with flunixin meglumine (1.1 mg/kg b.w., IV) for 5 min. (group E+FM1, n=4), 30 min. (group E+FM2, n=4), and 60 min. (group E+ FM3, n=4) after the administration of LPS. The study was meant to determine the concentration of testosterone (T), estrone (E1), estradiol-17b (E2b), total protein (TPR), albumin (ALB), zinc (Zn), cholesterol (CHOL), and aspartate aminotransferase (AST) activity in the seminal plasma of stallions. The administration of endotoxin had an influence on the T, E2b, E1 and TPR, ALB, CHOL concentration, as well as on AST activity in seminal plasma (in the period of the 24h-9th weeks after LPS administration). There has been a statistically significant decrease noted noted in the T, to 0.18 ng/ml (ΔT - 21.7%), and an increase in the E1 to 45,3 pg/ml (Δ E2b - 46.1%), E2 β, to 92.9 pg/ml, (Δ E2b - 45.6%), as well as in the: TPR, to 22.3 g/l (Δ TP - 76.9%); ALB, to 1.9 g/l (Δ ALB - 94.0%); CHOL, to 0.52 mmol/l (Δ CHOL - 21.0%), and AST, to 68.1 mU/109pl. (Δ AST-138.4%). The positive effects of treatment with flunixin meglumine were noted on T, E2β, ALB and CHOL concentrations in seminal plasma, especially of stallions group E+FM1.