Huntington's chorea is a degenerative disorder of the human brain characterized by a marked loss of intrinsic neostriatal neurons. This situation can be reproduced by kainic acid injection in the caudate nucleus. Activity of pars reticulata neurons ipsilateral to the injected neostriatum was studied in normal, control (saline-injected) and lesioned rats. They were identified by electrophysiological and histological criteria (Fig. 1). Results obtained in normal and control rats were very similar (Table I). As previously described, the mean frequency of these neurons was high. An important percentage (respectively 72.5 and 73%) and these neurons presented the characteristics of a regular firing pattern (so called "organized neurons"). Results obtained in kainic acid lesioned rats were significantly different (Table I). The mean frequency was lower and only 11% of reticulata cells remained organized after neostriatal lesion. This important dysfunction may be explained in various ways: The neostriato-nigral pathway's destruction involves both the inhibitory GABAergic tract and the excitatory substance P tract (GALE et al., 1978). Other inputs arising from many structures in the brain continue to exert their own action on SN neurons, resulting in an unbalance in the SN inputs. It is well known that the nigral dopamine influences the neuronal activity of pars reticulata neurons (Ruffieux et Schultz, 1980; Waszczak et Walters, 1983). Doudet et al. (1984 b) previously reported a dysfunction of neuronal activity of dopaminergic cells after striatal lesion. A disturbance in the electrical activity may induce a similar disturbance in the intranigral dendritic release of DA.(ABSTRACT TRUNCATED AT 250 WORDS)