Laboratory of Behavioral Sciences, National Institute on Aging, National Institutes of Health, Baltimore, Maryland 21224, USA. To identify the brain areas involved in central command, four monkeys were trained to attenuate the tachycardia of exercise while different brain sites affecting heart rate (HR) were simultaneously stimulated electrically. Among 24 brain sites located mostly in the limbic structures, we have identified four types of control systems that mediate cardiovascular and motor behavior during exercise. One system increases HR equivalently during both exercise and operantly controlled HR, whereas another increases HR during both tasks and abolishes operant HR control. In the third system, the effect of brain stimulation on HR is attenuated during exercise and during exercise with operantly controlled HR. The fourth system increases HR in both tasks, but its effect is significantly attenuated during operant HR control. We believe that this last system, which includes the mediodorsal nucleus, nucleus ventralis anterior, and cingulate cortex, plays a significant role in central command.