Milk is a food containing many biologically active substances that have an important place in the nourishment of newborns and adults. The transition of the drugs used in the treatment, as well as the environmental pollutants to milk cause a potential risk for consumer health as well as economical losses due to exceed of the legal limits of these compounds set by authorities. The transition of these chemicals to milk is complex; while active transport and passive diffusion play were found to have an important role. The transition abilities of the drugs into milk are defined by milk/plasma ratio. The M/P ratios of the drugs are affected by the composition of the milk and the physicochemical properties of the drug. The concentration of the drugs in the milk depends on the factors of the drug (protein binding, ionization, molecular weight, lipophilicity, drug-drug and drug-nutrient interactions) and organism (race, species, lactation period, parity, disease and nutrition). If the transition properties of the compounds of concern are known or able to be modelized in kinetic applications, it can be useful for preventing milk from drug residues.The success of mastitis treatment depends on the proper use of drugs and knowing of the behaviour of drugs in the milk.