A wide range of cognitive-behavioral sequelae, including memory deficits, results from hard metal disease in humans. Cobalt is a common component in the manufacture of hard metals and is a biologically active, toxic substance. This study examined the effects of cobalt exposure in rats. Results showed decreased exploratory behavior and a trend for higher-dose subjects to show decreased passive avoidance learning. No significant differences in active maze learning were found. These results indicate the value of further explorations of the cognitive-behavioral effects of cobalt exposure and suggest a number of methodological cautions.