Irradiation of the neonatal rat hippocampus reduces the proliferation of granule cells in the dentate gyrus and results in locomotor hyperactivity, behavioral perseveration and deficits on some learned tasks. In order to address the role of changes in stimulus salience and behavioral inhibition in animals with this type of brain damage, irradiated and normal rats were compared in their startle reactions to an acoustic stimulus. A portion of the brain of 10 rats was exposed to a fractionated total dose of 13 Gy during the first 16 days post partum. This procedure produced selective hypoplasia (91% reduction) of the granule cells in the hippocampal dentate gyrus. Other rats (N = 10) were sham irradiated. Sudden tones were presented to each adult rat at a rate of 1 every 30 s (spaced trials) during an initial 10-min session and 1 every 15 s (massed trials) during a subsequent session. Irradiated rats startled with a consistently higher amplitude than controls and were more likely to exhibit startle responses. These animals with hippocampal damage also failed to habituate to the startle stimulus and, under certain circumstances, showed potentiated startle responses after many tone presentations.