Nitrofuran derivatives, particularly nitrofurazone aind furazolidone are widely applied in a veterinary therapy. They show a wide spectrum of bactericidal properties and, what is still more important, are effective in relation to many species and strains of bacteria resistant to antibiotics and sulphamides (5, 13, 14, 15, 16, 24, 25, 27). Moreover, certain nitrofurans are also effective towards protozoa and some major viruses (2, 3, 4, 9, 12, 20). Among the preparations based on nitrofuranic compounds, particularly popular in our country is Endofuran, containing furazolidonum - 2.3%, nitrofuralum - 2.3%, natrium laurylo-sulfuricum - 0.32%, kalium bichromicum - 0.43, lactosum - 94.65%. This preparation is widely applied at control of diarrhoeal diseases of animals, and particularly of swine colibacillosis. However, the cases also are known, in which Endofuran applied in the doses recommended by the factory does not bring expected therapeutical effects. One of the reasons of this weak effectiveness may be probably - at least in some cases - too little amounts of the preparation administered to sick animals. Of it both home observations, proving e.g. that the double Endofuran dose would often result in a cure of sick animals, for which the doses recommended appeared to be ineffective, and the reports of some foreign authors, applying successfully to swine 25, 30, 50, 100, 150 and even 200 mg of nitrofuran compounds per 1 kg of body weight (1, 16, 17, 18, 26), can bear evidence. As compared with the above data, the maximal recomended therapeutic dose of Endofuran for piglets is with us 3 g per day, including only 138 mg of nitrofuranes (furazolidone + nitrofurazone), i.e. in the case of a piglet weighing 20 kg only 6.9 mg/kg of these compounds, what seems to be in fact too little. An increase of the obligatory Endofuran doses "by guess" or basing only on the data from scientific literature would be after all risky, the more that in composition of this preparaition, beside of nitrofarans, also other chemical compounds are contained. Changes or corrections of the Endofuran dosing can be done, like in the case of any other drug, only basing on concrete results of the respective experiments, taking into consideration, among other things, its tolerance limits in animal organism. In the Polish veterinary literature, beside of the report of Tereszczukowa and Zieliński, there is a lack of any research concerning this problem (23). In the above situation it seemed purposeful to undertake the present work, aiming at determination of effect of particular Endofuran doses, administered to healthy pigs for differently long periods, on clinically estimated general condition of the animals, physiological efficiency of their alimentary tract and sensitiveness in vitro of E. coli and Lactobacillus group rods to nitrofurazone, sulphatiazol and some antibiotics.