Serum levels of corticosterone and 18-hydroxy-11-deoxycorticosterone in the female rat at the high and low points of the circadian rhythm.

Serum corticosterone (B) and 18-hydroxy-11-deoxycorticosterone (18-hydroxy-DOC) levels were determined in female rats at the high (1800 h) and low (0600 h) points of the circadian rhythm. In order to carry out these studies, a rapid and accurate non-chromatographic radioimmunoassay method was developed for the measurement of 18-hydroxy-DOC in peripheral blood and similar methodology was used for the B assay. In quiescent rats both steroids were dramatically elevated at 1800 h as compared to 0600 h. The serum levels of B and 18-hydroxy-DOC determined at 1800 h fifteen minutes following stress did not differ significantly from the levels determined following a similar stress at 0600 h. There was a good correlation (r = 0.91) between the levels of B and 18-hydroxy-DOC and it appears that both steroids are regulated by ACTH.