The secretion of neurohormones from the crustacean X-organ – sinus gland system is controlled by environmental influences, light being the most conspicuous. Two sets of photoreceptors appear to mediate the influence of light on neurosecretion based on intracellular recordings from X-organ neurons and estimations of hormone release. Extra-retinal photoreceptors can initiate neurohormonal release from the eyestalk. Neurosecretory activity is also influenced by putative neurotransmitters. GABA is found in high concentrations in the medulla temninalis of the eyestalk and is released by stimulation, in a calcium-dependent manner. Diurnal variations occur in the amounts of eyestalk neurohormones, either those present in the eyestalk or released by electrical stimulation of the isolated sinus gland. Rhythm phases vary from one hormone to another. Neurohormones secreted in the eyestalk are also found in other regions of the central nervous system. Rhythms of neurosecretion are present both in the secretion in the isolated eyestalk and in eyestalkless animals, thus indicating that rhythmicity is a distributed property of the neurosecretory system.