The mammalian suprachiasmatic nuclei contain an endogenous circadian pacemaker. A quantitative autoradiographic tracer method that measures l-[1- 14C]leucine incorporation into protein was used to determine whether the overall rate of protein synthesis in the nuclei varies in a circadian fashion. Unlike the robust circadian rhythms of electrical activity and energy metabolism previously recorded from the nuclei at the two points sampled, protein synthesis remained constant.