Plasma corticosterone levels were determined in serial samples obtained from 3 groups of rats: 1) starved, 2) cold-treated (4 degrees C), and 3) starved and cold treated (4 degrees C). Starvation resulted in an increase of plasma corticosterone on the 3rd day and the increase henceafter was linear with the number of days of treatment up to day 13 whereas upon cold exposure, the plasma corticosterone levels first increased on day 1 up to day 3, and then declined on day 9. Cold and starvation in combination led to a greater elevation of plasma corticosterone level than cold or starvation alone. It is concluded that the increase of plasma corticosterone during starvation or cold may be related to altered metabolism under such circumstances, and that the effect of cold and starvation are synergistic and may be mediated by different neural mechanisms.