Using an automated method, reduced auditory startle responsiveness was detected in Swiss Webster mice fed excess aluminum in diet (1000 micrograms Al/g) as compared to controls (7 micrograms Al/g). Excess aluminum diets were fed either from conception to weaning (developmental group) or from conception and continuing to 52 days of age (continuous group). At 22 days of age, lower response amplitudes were seen in the aluminum exposed groups than in controls. At 52 days of age, more rapid reduction in response amplitude across initial trial blocks was noted. Effects were more prominent in the continuously exposed than the developmentally exposed group at 52 days of age. Reduced auditory startle responsiveness is interpreted as part of a pattern of subtle motor impairment and marginally reduced activity and responsiveness in mice fed excess aluminum in diet.