An autoradiographic study of the development of 26 nuclei in the rat hypothalamus was made by injecting pregnant rats on different days of pregnancy with tritiated thymidine. The offspring were killed postnatally and the day of gestation on which peak percentages of heavily labeled neurons occurred was considered to be the birthdate of that nucleus. Postnatal animals for the first nine days were injected and were also included in the study. Laterally placed nuclei were found to arise on the fourteenth day of gestation while medial nuclei, in general, arose on day 16. Final cell divisions occurred over a period of time but the majority took place on the days indicated. Apparently a lateromedial gradient exists in the hypothalamus, and possibly a dorsoventral gradient.