The magnetic resonance images of 67 healthy subjects aged 4-50 years were studied for differences in general signal intensity between the different brain structures, the frequency of focal intensity changes in the brain, and variations in size of the cerebrospinal fluid (CSF) spaces. In adults over 25 years of age the thalamus gave lower signal than the putamen or caudate nucleus. Definite periventricular high signal was found in the white matter of one third of subjects of all ages. Small (< 5 mm in diameter) high signal foci were found in the cerebral white matter on T2-weighted images in 27% of subjects (20% of healthy children and adolescents and 34% of adults). They gave high signal on both short and long echoes in 11% of children and adolescents and in 22% of adults; 51% of all foci gave high signal with both echoes. This does not support the hypothesis that they are caused mainly by enlarged Virchow-Robin spaces. Of the high signal foci on T2-weighted images, 86% were in watershead areas. Two foci were found in one subject in the periventricular watershed area (beside the tips of the frontal horns) and they were never seen in the other deep white matter regions. In healthy, relatively young subjects with no known risk factors, high signal foci other than Virchow-Robin spaces, were common; neither their prevalence nor their number correlated with age in this series. A few slightly large sulci were found in some adults.