Aim: To describe histochemically and immunohistochemically a breast tumor presented in a pregnant rat given Cycloheximide to examine its teratogenic and embryotoxic effect on the embryos.Methods: Cycloheximide was injected in pregnant Wistar rats at a dose of 3 mg/kg.b.w. on both 10th and 11th gestational days. In one of the rats, a large breast tumor developed rapidly. Histochemical staining with Hematoxylin-Eosin and Masson Trichrome and immunohistochemical identification with mouse monoclonal antibodies: a) Estrogen Receptor A and b) Estrogen Receptor B was performed.Results: Analysis with A-receptor and B-receptor showed that the breast tumor which was developed after treatment with Cycloheximide was malignant. Positive immunohistochemical reaction was evident especially with A-receptor indicating the malignancy of the tumor.Conclusions: Cycloheximide is a known toxic and teratogenic agent and potentially a carcinogenic drug. Thus it should be used with extreme caution as a pesticide.