This investigation demonstrates the importance of the direct hippocampoanterior thalamic component of the postcommissural fornix in the control of general locomotion and active avoidance. Transection of anterior thalamic afferents from the hippocampal formation (subicular cortex), at the point where they exit from the fornix posterior to the septum, is sufficient to enhance bidirectional active avoidance acquisition and increase general activity. This transection may also interrupt fibers to or from other thalamic nuclei and the anterior septum. However, destruction of connections of the anterior septum with the hippocampus, habenula, and thalamus by transection in the coronal plane anterior to the descending fornix columns, without damage to the subiculothalamic fibers, increases general activity levels without affecting active avoidance behavior. The activity increase in this case resembles that seen after septal lesions rather than that seen after hippocampal lesions. Thus, destruction of a single fornix component contributing afferents to the anterior thalamic nuclei reproduces at least part of the hippocampal syndrome. This suggests that these fibers contribute significantly to the control of these behaviors and may mediate active avoidance changes resulting from hippocampal and fornix damage.Previous investigations have demonstrated similarities in the behavioral effects of hippocampal and septal lesions. Damage to these areas is known to elevate activity levels (reviewed by Douglas, 1967;Kimble, 1968), facilitate active avoidance acquisition (Isaacson, Douglas & Moore, 1961;King, 1958), and impair performance on a differential reinforcement of low rate schedule (Clark & Isaacson, 1965;Ellen, Wilson, & Powell, 1964). Many of these effects are replicated by selective transection of the major fiber systems innervating the hippocampus and septum, particularly the reciprocal connections of the hippocampal formation and septum in the fornix-fimbrial complex (MacDougall & Capobianco,