In the first experiment, plasma corticosterone levels were determined for mice which had been either left undisturbed or exposed to one of three test apparatus which differed from the home environment to varying degrees. For each successive increase in the degree of environmental change experienced by mice, there was a corresponding significant increase in mean plasma corticosterone level. Experiment 2 shows that the handling which mice received while being placed into the apparatus did not contribute to the corticoid elevations. Mice, which had been picked up and immediately returned to the home pan, exhibited corticosterone concentrations which did not differ from resting levels. Overall, the study demonstrates that, contrary to previous reports, changes in corticoid levels can sensitively reflect the intensity of stimulation to which an organism is exposed.