A knowledge of the acute systemic effects of materials that come into contact with body tissues and fluids is necessary to assess their clinical safety. Here we describe the use of bioradiotelemetry to record the heart rate, body temperature, and motor activity of freely behaving rats in conjunction with weight gain, food, and water consumption after administration of the bacterial endotoxin lipopolysaccharide and the heavy metal nickel chloride. The results of our study reveal that body temperature change was the more sensitive index of the two physiological parameters tested. Core body temperature can be monitored diurnally or nocturnally depending on whether a hyper- or hypothermic response, respectively, to the test solution is anticipated. Weight gain was the most sensitive index of all parameters monitored, but its use as an index requires that the test substance be administered nocturnally. Monitoring physiological parameters offers a complementary assessment of a systemic response, but these markers are not superior to conventional indices such as reduced weight gain. The use of two diverse toxic substances in the present study illustrates that test conditions are dictated by the nature of the systemic response induced by the substance tested.