Serial ultrasonography was performed to measure the normal appearance of uterine during estrous cycle and to determine whether the unterine appearance was related to the sex hormone, progesterone and estrogen. The uterine appearances, shape, diameter and echogenicity were daily monitored with ultrasonography in 9 Miniature Schnauzer dogs undergoing II estrous cycles. During proestrus and estrous, the uterus became hypoechoic but developed hyperechoic luminal echo. In the longitudinal view, the shape of the uterus occasionally changed from rectangular to coiled or serpentine, compared to other stages of the cycle. The diameter of the uterus during proestrus and estrous was larger (range: 0.600.86 mm) than other stages (range: 0.480.62 mm) of the cycle. The rising estrogen concentrations (range: 14.5116.86 pg/ml) in plasma during proestrus correlated with changes in the uterus (p<0.05). Progesterone concentrations were 0.080.15 ng/ml at the onset of proestrus, but rose 1.061.26 ng/ml at the end of proestrus. There was no relation to progesterone concentration from onset of estrus (p>0.05). There was dramatical changes in normal uterus and sex hormone during estrous cycle. Especially, the appearance, shape and diameter of uterus were related to plasma estrogen concentration during proestrus, correlated with other stages of the cycle.