This work was undertaken to investigate changes in milk constituents of Japanese Corriedale ewes during 3 months of lactation after lambing, analysing their milk composition. The sheep tested were 7 Corriedale ewes of 3 years of age kept at the Chugoku Stock Breeding Farm of the Department of Agriculture and Forestry and the milk samples for analysis of the composition of ewes' milk were taken by hand milking on the next day of lambing and at 10-day intervals during 3 months after lambing. The results obtained were as follows: (1) Fat-, protein-, ash-percentage, Sp. gr. and acidity of ewes' colostrum showed considerably higher values than those of normal milk, but only percentage of lactose a lower value. (2) Sp. gr., acidity and percentage of protein of ewes' milk had tendencies of gradual increase, while percentages of fat and lactose decreasing tendencies and percentage of ash remained constant in course of 3 months' lactation. (3) It seems that protein percentage in ewes' milk is inversely proportional to their milk yield, but no definite relations of each between milk yield and percentages of fat, lactose and ash exist.