The present study investigated the hypothesis that changes in the frequency of specific behaviors, not assessed by the traditionally employed global measures of locomotor activity, may occur following exposure to cadmium. Two groups of newborn male rat pups were treated orally with either 1.0 mg/kg CdCl2 for 14 days or 10.0 mg/kg CdCl2 for one day followed by 13 days of the treatment vehicle only. A control group received only the vehicle. At 50 days of age, animals were observed in their home cage for 12 hours and the amount of time spent engaged in each of eight separate categories of behavior was recorded. Subsequently, exploratory behavior in an open field was assessed. The single large dose of cadmium resulted in weight loss and a significantly slower growth rate significantly increased rearing in the home cage and significantly decreased inner square exploratory behavior in the open field. Implications of emotional hyporeactivity and physiological explanations which may account for the differences observed between the two treatment regimens are discussed.