Behavior and corticosteroid response of Maudsley reactive and nonreactive rats in the open field and forced swimming test.

Maudsley reactive (MR) and nonreactive (MNRA) male rats were tested in the open field and in the forced swimming test. MR rats defecated more and were less active than MNRA rats in the open field. MR rats also defecated more, but were more immobile in the forced swim test. The two strains did not differ significantly in basal corticosteroid levels or corticosteroid levels in response to either test. These results suggest that the increased defecation rates exhibited by MR rats in various tests are not indicative of a general increase in "emotionality" or stress level.