AbstractThe effect of different concentrations of fluoride (as sodium fluoride) on the enzymes lactate dehydrogenase (LDH, EC 1.1.1.27) and creatine kinase (CK, EC 2.7.3.2) in rabbit plasma was investigatedin vivoandin vitro. Rabbits were dosed orally, for 30 days, with fluoride at 2, 10 or 20 mg per kg body weight as sodium fluoride. LDH and CK levels were determined at 10‐day intervals during the treatment. Fluoride, depending on the applied dose, inhibited, had no effect or stimulated the investigated enzymesin vivo. On the other hand, fluoride had no effect on the activity of LDH and CKin vitr