The critical points of technic for the prepn. of radiopotassium autographs of rats'' brains are discussed; results can be obtained despite the fact that this isotope has a short half life and enters brain tissue very slowly. It is found that the distribution of K42 in the brains of normal rats is nearly homogeneous except for slightly greater concn. in the cerebral and cerebellar cortex, the basal ganglia and the roof of the 4th ventricle.