Doxicycline is a structural isomer who meets the European reference standard (Ph.Eur. III 1997:0272) and respect EU Regulations for M.R.L. The good solubility permits high tissular absorption and distribution after administration in drinking water. The aim was to ascertain the clinical effects and tolerance consecutive to risen doses administration, as a part of safety study for this antibiotic in poultry in the respect of current drug testing methodology. The safety study revealed: good local and general tolerance to therapeutic doses (10 mg x kgb.w.-1) and to x2 dose, diarrhoea in lots which received x3 and x5 the therapeutic dose. Comparatively with the Control lot, haemoleucogram doesn’t suffer evident changes the registered values being between the references limits (exception of leucocitary formula). Also it was found an increased creatinin concentration consecutively to x3 and x5 greater doses administration and marked increasing of ASAT level and limited for ALAT’s, comparatively to control lot in the case of E4 lot, but light to reference values. Macro and microscopy revealed for the liver samples: hepatomegaly, diffuse hepatic degenerescence, nuclear heterochromatinization, vacuolar degenerescence; renal changes in nefrons and renal corpuscles, light splenomegaly, caecal sacs distension and brown-yellowish gaseous content to lots E3 and E4.