Experimental findings have suggested the possibility of a functional relationship between the basal ganglia and the hippocampus. Previous research has revealed a predominantly inhibitory action of the caudate nucleus (CN) and an excitatory effect of the globus pallidus (GP) on electrically induced hippocampal afterdischarges (HAD). The effects of electrolytic destruction of the CN on the threshold and duration of HAD has been studied in the 'encéphale isolé' cat. The threshold and duration of HAD was also studied following conditioning stimulation of the CN in animals in which the inner segment of the globus pallidus (GPi) and medial septal nucleus (MSN) had been destroyed. Following CN lesions, the hippocampal excitability threshold underwent a significant reduction, while the duration of HAD appeared to be increased. Following destruction of the GPi and MSN, the threshold and duration of HAD exhibited no change following conditioning stimulation of the CN. The results reveal a tonic inhibitory effect of the CN on the hippocampus and suggest that a strio-pallido-septal pathway is the anatomical substrate for the effect.