Investigated the effects of bilateral lesions of the anterodorsal caudate nuclei on reactivity in 5 experiments with male albino Sprague-Dawley and Wistar rats. In the open-field apparatus, Ss with caudate lesions were more active, showed increased thigmotaxism, defecated more, and groomed less. Exp II showed that in a nonstressful situation Ss with caudate lesions defecated no more than those with cortical lesion or unoperated Ss. Under stressful conditions Ss with caudate lesions defecated significantly more than the control Ss. Exps III, IV, and V indicated that Ss with lesions of the caudate nucleus were hyperresponsive to conditions of illumination and food deprivation. It is suggested that the heightened reactivity of the caudate-damaged Ss reflects the role of the caudate nucleus in the modulation of CNS arousal mechanisms. (52 ref.) (PsycINFO Database Record (c) 2006 APA, all rights reserved)