The main purpose of this study was to investigate the effect of thyroxine and PTU in ascitic Ehrlich tumor cells. Tumor was implanted in 30 female mice distributed in three groups: treated with PTU, treated with thyroxine and control. Each group received an intraperitoneal injection of neoplastic cells, pre-incubated with sterile solutions of PTU, thyroxine and distilled water, respectively. On the fifth and seventh days after inoculation, animals received an intraperitoneal injection of the respective solutions. On the tenth day after inoculation, animals were sacrificed. Volume of ascitic liquid, number of neoplastic cells/ml and percentage of viable cells were determined. Ascitic liquid smears were carried out for tumor cytological evaluation. There was no difference among groups regarding ascitic liquid and as for the number and viability of tumor cells. However, cells under the effect of thyroxine presented significantly larger mean of nuclear diameter, size and number of nucleolus organizer regions. In this group, there was a predominance of clear, round cells with abundant eosinophilic and very vacuolated cytoplasm with little defined edges. Under the PTU effect, tumor cells were small with hyperchromatic nucleus and the same number of NORs as the control group. It was concluded that PTU and thyroxine have not changed the number and viability of cells after 10 days of tumor inoculation but they changed significantly cell characteristics. Whilst thyroxine increases cell size and the number of NORs of ascitic Ehrlich tumor cells, PTU causes an opposite effect