The effectiveness of a recently developed immunotoxin, 192 IgG-saporin, was evaluated for making selective lesions of subgroups of basal forebrain cholinergic neurons. Following a pilot series of injections into the nucleus basalis magnocellularis to establish the effective dose for intraparenchymal lesions, separate groups of rats received injections of the immunotoxin into the septum, into the diagonal band of Broca or into the nucleus basalis magnocellularis. The lesions produced extensive and effective loss of cholinergic neurons in the discrete areas of the basal forebrain, as identified by loss of cells staining for acetylcholinesterase and p75NGFr, with a parallel loss of acetylcholinesterase staining and choline acetyltransferase activity in the target areas associated with each injection site in the dorsolateral neocortex, cingulate cortex and hippocampus. The selectivity of the lesion for cholinergic neurons was supported by the lack of gliosis and sparing of small to medium-sized cells at the site of injection of the toxin, including the glutamate decarboxylase immunoreactive cells that contribute to the septohippocampal projection. In spite of the extensive disturbance in the cholinergic innervation of the neocortex and hippocampus, immunotoxin lesions produced no detectable deficit in the Morris water maze task in any of the lesion sites within the basal forebrain. By contrast small but significant deficits were seen on tests of nocturnal activity (septal and nucleus basalis magnocellularis lesions), open field activity (septal and diagonal band lesions), passive avoidance (nucleus basalis magnocellularis lesions) and delayed non-matching to position (septal lesions). The results indicate that the 192 IgG-saporin provides a powerful tool for making effective lesions of the basal forebrain cholinergic neurons, and that the behavioural sequelae of such lesions warrant further detailed investigation.