The effects elicited in the hypothalamo-neurohypophysial system by labyrinthine stimulation have been studied histologically in guinea-pigs using methods for the staining of neurosecretory material. With reference to unstimulated controls, in intact animals killed after rotational stimulation an obvious reduction of this material was seen to occur both in neurohypophysis and supraoptic nucleus. Control experiments in bilaterally labyrinthectomized animals yielded no evidence of reduction, either when rotated or when not rotated. Thus, causal relationships may be held to exist between the activation of vestibular receptors and the functional changes in the neurosecretory system. The neutral paths mediating the effect and the functional significance thereof are briefly discussed.