The present study was designed to localize the effect of hypophysectomy on brain RNA content at a topographical level. For this purpose, a brain dissection method, based on morphological dif-ferences between various brain areas, was developed. In these areas, total cell RNA was measured and expressed as the RNA/DNA ratio. The highest ratio was found in the cortex cerebri (ca. 2.15) and hippocampus (2.21), whereas cerebellum (0.42) and bulbus olfactorius (0.95) showed the lowest values. Hypophysectomy reduced the RNA content mainly of brain-stem structures (ca.–24%), i.e., the thalamus, hypothalamus, mesencephalon, and medulla oblongata. Nevertheless, a small but significant reduction was found in the rostral cortex. In view of previous studies, it is suggested that this disturbed RNA metabolism in brain-stem structures could have a bearing on the impaired avoidance performance of hypophysectomized rats.