Mice were subjected to three types of acute stress (cold, forced swimming and tail hanging) in order to investigate the effects of stress on the motor activity circadian rhythm. This rhythm was studied using the cosinor method and spectral analysis. A statistically significant decrease in the amplitude and the power content of the circadian harmonic was found after stress application. These decreases could be due to a desynchronization of the circadian oscillators which drive the rhythm. The use of the power content of the circadian harmonic is proposed for the detection of the alterations due to stress.