The degree of the contralateral sensory neglect in rats with unilateral 6-OHDA lesions of substantia nigra was assessed by a conditioning procedure, employing lateralized CS. In the first experiment visual neglect (revealed by failure of the visually elicited placing reaction contralateral to the lesion) was shown to be accompanied by slower acquisition of a brightness discrimination task. The impairment was due to ipsilateral turning tendency rather then to visual deficit, however, since monocular relearning yielded equal savings with the ipsilateral and contralateral eyes. The second experiment showed that rats anesthetized with urethane reacted to noxious skin stimulation contralateral to the lesion with shorter-lasting EEG arousal than to ipsilateral stimulation of the same intensity. The electrophysiological asymmetry could be compensated by classical conditioning, i.e. by pairing habituated tactile stimuli with noxious tail shock. The conditioned arousal reaction could be elicited with the same efficiency from the neglected and intact body surface. It is concluded that neglect is due neither to a sensory nor to a motor failure, but that 6-OHDA lesions of substantia nigra in one hemisphere reduce the arousing efficiency of unconditioned stimuli and interfere with sensorimotor integrating mechanisms on the side contralateral to the lesion. Compensation of the neglect by conditioning indicates that the role of the nigrostriatal system can be partly substituted by other circuits.