A stress-induced increase in noradrenaline (NA) release was measured by intracerebral microdialysis in the hypothalamic paraventricular nucleus of freely moving Wistar-Kyoto rats at three different ages (6, 18 and 24 months). NA levels in 20-min dialysate samples were measured by high-performance liquid chromatography with electrochemical detection. Microdialysis sampling was done at the baseline during a 20-min immobilization stress and for the next 100 min. Basal NA release was not significantly different in the three age groups. The immobilization stress increased NA levels (247, 197 and 234% of the baseline for the 6-, 18- and 24-month animals, respectively) which was not significantly different in the three groups. In the two younger groups NA returned to the baseline in the first sample after the end of the stress (t = 40 min) whereas in the 24-month group it remained significantly higher for longer (until t = 60 min). Stress-induced release of hypothalamic NA thus appears to be prolonged in old rats.