A new method was used to measure stress hormone levels in mice. The study found that levels of the hormone corticosterone in the blood and brain of mice increased significantly around day 12 after birth, peaked around days 18-20, and then decreased by day 30. This pattern was different from what was previously thought due to the limitations of the old measurement method. The changes in corticosterone levels in the brain followed a similar pattern to those in the blood. Genetic differences only affected stress-induced corticosterone levels after the first week of life. These findings help us understand how stress hormones develop in young mice and how they may affect the development of other tissues in the body.