An analysis of the gross composition of milk in five inbred strains of mice (NZB, CBA/H, BALB/c, C57BL/6, XLII) was conducted for two different days during the lactating period (6th and 9th day after parturition). The quantity of milk and the concentration of proteins, lactose and lipids were measured. Results show a significant effect for day on the quantity of milk, but not for strain. Day effect was also observed for proteins and lactose; however modifications in the concentration of lactose and lipids from the 6th to the 9th day were not identical across strains. Also, a strain effect was present for proteins, lipids and lactose.