A very simple technique for measurement of rat serum corticosterone with radiostereoassy kit using 75Se-cortisol was devised. Only 25--50 microliter serum was required for this assay. The serum sample was first diluted to 300 microliter with water and heated in a glass tube to denature the endogeneous cortisol binding protein. Then an aliquot of the sample was transferred into an assay vial containing 75Se-cortisol, cortisol binding protein and adsorbent granules. After rotated, an aliquot of the supernatant was taken for counting. High level of rat serum corticosterone was measured with satisfying accuracy by dilution with water. Serum corticosterone levels in 7 normal female Sprague-Dawley rats (7 weeks old, Ca 200g body weight) at 2.00 pm were 29.5 +/- 12.2 microgram/dl (mean +/- SD). The levels were lower and higher in rats treated with dexamethasone and with 1-18 ACTH, respectively.