Suppression of behavior accompanying increased ACh synthesis in the brain might account, at least in part, for the preferred use of lithium in antimanic therapy. Three experiments using rats as subjects were designed to test hypotheses about relationships among lithium, ACh synthesis and behavior. Experiment 1 established that hyporeactivity and greater exploratory behavior occurred in animals under LiCl treatment conditions shown to stimulate cholinergic activity in brain. Experiment 2 provided evidence of significant differences between controls and animals on the LiCl diet. Groups tested after 1 or 2 days of LiCl showed the decrease in reactivity to successive presentations of a loud auditory stimulus which characterizes the normal process of habituation. Groups tested after 5 or 10 days of liCl showed no evidence of habituation, their reactivity throughout the period of stimulation being at a level attained by the other groups when habituation reached its final asymptote. Experiment 3 established that effects of LiCl treatment were not manifested in all aspects of behavior: there was no evidence of impairment of motor activity or coordination; no analgesia; no impairment in sensory input nor in acquisition of new behaviors. The effect of the LiCl treatment was not complete inhibition but instead suppression of reactivity to environmental stimulation under conditions shown previously to increase cholinergic activity in brain.