The present work aimed to evaluate deltamethrin toxicity and to estimate the pesticide residue in milk and serum of dairy cows and buffaloes after different periods of deltamethrin withdrawal. 240 blood and 240 milk samples were collected from dairy cows and buffaloes in El-Behaeira Governorate farms during summer 2020 before spraying with deltamethrin (50 mg/L) with a quantity of (1 ml/ liter water) two times a week apart and after cessation of spraying for 1, 7, and 21days. The obtained results proved that deltamethrin resedues were not detected in milk and serum  before spraying and after 7 and 21 days of withdrawal. However, Deltamethrin residues exceeded the permissible limit in cow's milk and in buffaloes' milk after one day of withdrawal. All cow's buffaloe's milk samples were contaminated with deltamethrin after one day of withdrawal. Deltamethrin residues ranged from 0.05 to 0.10 ppm in cow's milk and from 0.07 to 0.10 ppm in buffaloes milk after one day of deltamethrin withdrawal. Cows and buffaloes exhibited a significant elevation in serum alanine aminotransferase, aspartate aminotransferase activities, urea, creatinine, and lipid peroxide (malondialdehyde) concentrations while the total protein, albumin, globulin, and antioxidants activities declined significantly after 1 day and 7 days of withdrawal. However, all biochemical parameters significantly improved after 21 days of treatment cessation.