There have been no reports on how the light-dark changes determine the locomotor activity of animals in the group of high reactivity (HR) and low reactivity (LR
(Vievpoint France). According to our results, we noted that a change of the experimental conditions from light to dark involves an increase in the locomotor activity in rats and opossums of the HR group, while there is no effect on the activity of the rats and opossums in the LR group. Locomotor activity in the HR rats, both in the light and dark conditions is characterised by a consistent patternof change -higher activity in the first stage of the recording and a slowdown (habituation) in the second phase of the observation. The locomotor activity of the opossum, during both light and dark conditions, was observed to be at a consistently high level compared to the rats. (Folia Morphol 2013; 72, 4: 300-305)