When a rat is first placed in an overcrowded cage, it will likely enter the stage of the general adaptation syndrome.

alarm

Explanation:

The physiological changes that take place in the body in response to stress occur in a three-stage process which is known as the general adaptation syndrome. 

The three stages are as follows:

1. Alarm stage: This constitutes the initial reactions of the body in response to stress due to the release of the stress hormone - cortisol. 

2. Resistance stage: This is the stage where the levels of cortisol lower and the body repair itself to bring itself back to the initial stage before the reaction that took place at the alarm stage. 

3. Exhaustion stage: This is the stage where the body has lost the capability to cope up with prolonged stress.

Therefore, the first reaction in an overcrowded cage will be the alarm stage.