The plasma catecholamine stress response is characteristic for a given animal over a one-year period.

The same male and female rats with indwelling jugular catheters were stressed (immobilization) on two occasions at the ages of 3 to 4 and 15 to 16 months. Plasma levels of the catecholamines (CA) norepinephrine (NE) and epinephrine (E) were determined before and during stress at each session. During stress, plasma NE and E levels increased markedly. At the younger age, female animals showed markedly higher CA levels than male animals. After one year, stress CA levels were higher for both sexes, but had increased markedly in male and little in female animals. A positive correlation was found between the plasma CA stress response of individual animals at both sessions; this correlation was stronger for males and NE. This indicates that "high" responders usually remain high responders, "low" responders remain low and intermediate responders intermediate. In conclusion, the plasma CA stress response increases markedly in male, but little in female rats over the period of 1 year and the relative magnitude of the individual stress response remains a characteristic of each given animal.