Abstract: The plasma corticosterone level of rats was determined and used as acriterion for magnitude of stress response in this model. The rats wererandomly divided into three groups: C, control group; R, regular lightgroup; Ⅰ, irregular light group.The experiment was successively carried out in three stages:Ⅰ. adaptation (7 days); Ⅱ. development of stress(7 days);and Ⅲ. psychological induction of stress (0-24 hrs). Stage Ⅰ: all animalswere adapted to the cage and other environmental cues. Stage Ⅱ. in groupR electric shocks were applied to the tail regularly with a fixed intervalafter the lights were given. In group I the same amount of light and ele-ctric shocks were irregularly applied. In group C only irregular light wasgiven. Stage Ⅲ. all tail shocks were omitted and the same protocol oflights were used in different groups. The results showed that the plasmacorticosterone levels in the R and I groups were significantly higher thanthose in the C group (p<0.01.StageⅡ: R,24.9+\-5.8; Ⅰ,34.9+\-2.0;C,11.7+\-1.7. Stage Ⅲ. R,24.1+\-3.3; Ⅰ, 33.9+\-2.5; C,13.1+\-5.0. In every group n was 9 and the unit was ug/dl.), there was a signi-ficant difference between stress responses of R and I groups, both at stageⅡ and Ⅲ (P<0.05).

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