Metastatic patterns of fibrosarcoma (FS) and Lewis lung carcinoma (LLC) cells transplanted into mouse uteri of various reproductive stages, were investigated. Tumour cells were infused non-surgically into the lumen of 3-day post coitum pregnant uterus or into non-pregnant uterus of known estrus stage. The fate of these tumour cells was studied histologically on days 2, 5 and 10 post treatment. No significant difference in the metastatic patterns of the FS or the LLC cells was seen between the non-pregnant uteri of various estrus stages. FS cells in few cases of non-pregnant uteri displayed the tendency to migrate and grow outside the myometrium without colonizing in the endometrium, but in pregnant uteri they colonized within the endometrium. LLC cells in the non-pregnant uteri promptly metastasized to distant organs like liver and lung; but those in the pregnant uteri rarely metastasized to other organs. These observations imply that the metastatic patterns of uterine tumour might depend on both the physiological state of the uterus and the tumour cell type.