Rats were tested for 3 min per day, for 4 successive days, in an open field apparatus, 20 min after injection of either lithium chloride (2 mEq/kg) or physiological saline. In the first experiment, the open field was illuminated with moderate white light for some rats (stress condition) and dim red light for others (non-stress condition). In the second experiment, some rats received an electric foot shock 5 h before each open field test (stress condition) and others received no foot shock (non-stress condition). In both experiments, lithium significantly reduced rearing behavior in the stressed, but not the unstressed, rats. Lithium also reduced horizontal locomotion, but this effect appeared in both stressed and unstressed rats. In the second experiment, defecation was measured, and it was found that lithium-treated rats defecated less than saline-treated rats in the first session, but not in subsequent sessions, due to habituation which occurred for the saline rats.