AbstractRecent evidence indicates that the arcuate nucleus of the hypothalamus is involved in the control of gonadotrophin secretion. In the present work the neurons of the arcuate nucleus were studied in normal and castrated adult male rats with the electron microscope.The neurons of normal animals appear quite ordinary in many respects and contain the same types of organelles that are common to neurons in general. In addition, however, many neurons contain structures resembling the chromatoid bodies commonly found in developing spermatids. There are also some peculiar features of nuclear structure such as nucleolar association with the nuclear envelope and the fragmentation of the nucleolus.Arcuate neurons of castrated animals show two variations, the most obvious of which is the occurrence of whoreled formations of closely apposed concentric cisternae of the smooth endoplasmic reticulum. These whorled bodies appear in several different forms. The simplest consist of complete uninterrupted cisternal rings. Variations from the simple form involve anastomoses between adjacent cisternal shells, areas in which the regular layered structure is obliterated by confluence of cisternal rings, areas of irregular spacing in which varying amounts of cytoplasm are trapped between adjacent rings, and the presence of semicircular incomplete outer rings. As many as five whorled bodies have been observed in a single cell. The whorled bodies in such a group are continuous with one another and with the common endoplasmic reticulum.Whorled bodies appear four days after castration, and by two weeks about one tenth of the neurons exhibit them. Very few whorled bodies appear in castrated animals that received daily injections of 2 μg testosterone propionate per gram body weight. Castration produced no obvious qualitative changes in the ependymal cells or their processes.The second variation resulting from castration is the presence of large populations of dense core vesicles in some of the neurons of the arcuate nucleus. Dense core vesicles are sparse in the neurons of normal animals. The significance of these variations is discussed.There is evidence to suggest that the whorled bodies may be an indication of enhanced synthetic activity following release from gonadal steroid inhibition. In any case, the whorled bodies label the cells in which they occur as direct links in the brain‐pituitary‐gonad axis.