The role of the developing cholinergic basal forebrain system on cognitive behaviors was examined in 7 day-old rats by giving lesions with intraventricular injections of 192 IgG-saporin or saline. Rats were subjected to passive avoidance on postnatal days (PND) 22-23, water maze testing on PND 50-60, and a open-field test (in which reactions to spatial and object novelty were measured) on PND 54. Behavioral effects of the lesions were evident only in the open-field test with 5 objects. Unlike controls, the lesioned rats did not detect a spatial change after a displacement of 2 of the 5 objects. Control and lesioned rats, however, showed comparable novelty responses to an unfamiliar object. Lesion effectiveness was confirmed by 75% and 84% decreases in choline acetyltransferase activity in cortex and hippocampus. These results suggest that the developing cholinergic system may be involved in spatial information processing or attention to spatial modifications.