The total volumes and relative quantities of grey and white matter have been measured in sixty-five normal male and sixty-five normal female cerebral hemispheres. Fixed hemisphere volume was found to fall linearly at 3.5% per decade for men from 641 ml at the age of 20 to 463 ml at the age of 100. For women the decrease was 1.9% per decade from 531 ml at 20 years to 462 ml at 100 years. After correction for the effects of fixation and for the secular increase in brain size, it was concluded that mean hemisphere volume remained roughly constant between the ages of 20 and 50 years (558 ml for men, 474 ml for women). After the age of 50 the mean volume in both sexes fell at about 2% per decade. The ratio of the volumes of grey to white matter was the same for the two sexes at all ages. Its mean value was 1.3 at the age of 20, falling to 1.1 at the age of 50, then rising steadily to over 1.5 at 100 years. It is impossible in practice to correct these measurements for the effects of fixation or secular change. Fourteen hemispheres from thirteen elderly female dements were also measured. The total volume was 18% lower than for an age-matched group of normals, but the ratio of grey to white matter was identical.