Deprivation of magnesium ion in the diet caused a significant decrease in whole brain acetylcholine content. At the reginal level, the most pronounced effect was seen in the corpus striatum, diencephalon and cerebellum. The rate of depletion and recovery varied from region to region, and what appeared to be a return to normal acetylcholine in whole brain was the total of regions either higher or lower than control. The data presented here suggest that a positive relationship exists between the lowered cholinergic activity in the corpus striatum and increased nocturnal spontaneous motor activity.