The total neocortical volume delimited between precise anatomical features was measured. Serial frontal sections cresyl-violet stained were measured with plainimetric methods and the total volume evaluated at 5, 10, 30, 60 and 180 days. The total volume was used to obtain the total number of neurons and glial cells at each age using our quantitative data (published in our previous papers). The evolution of the cellular densities were studied in the sensory and motor areas 10--4--3 and 2 (Leuba and coll. 1977) and in the visual and auditory areas 17--18a--18--41 and 20 (Heumann anc coll. 1977) in the mouse cerebral neocortex. 1. The total neocortical volume increases rapidly between 5 and 10 days, less between 10 and 30 days, slightly decreases at 60 days and reincreases at 180 days. 2. Between 5 and 30 days, the loss of cortical neurons is of about 30% by neuronal death. 3. The total number of glial cells doubles between 5 and 180 days. The increase is progressive through the development.