Partial lesions were made with kainic acid in the interpeduncular nucleus of the ventral midbrain of the rat. Compared with sham-operated controls, lesions significantly (p < 0.25) blunted the early (<60 min) free-field locomotor hypoactivity caused by nicotine (0.5 mg kg(-1), i.m.), enhanced the later (60-120 min) nicotine-induced hyperactivity, and raised spontaneous nocturnal activity. Lesions reduced the extent of immunohistological staining for choline acetyltransferase in the interpeduncular nucleus (p <0.025), but not for tyrosine hydroxylase in the surrounding catecholaminergic A10 region. We conclude that the interpeduncular nucleus mediates nicotinic depression of locomotor activity and dampens nicotinic arousal mechanisms located elsewhere in the brain.