Mature female brush-tailed possum (Trichosurus vulpecula Kerr) were captured locally and kept in the grounds of the Department of Zoology, University of Adelaide. Animals were anaesthetized by sodium pentobarbitone (Abbott). Glucosesaline (5% glucose in 0·9% NaCl solution) was given i.v. as required. The left adrenal gland was exposed and cannulated by a modification of the method of Bush & Ferguson (1953) for sheep. Subsidiary vessels were tied off and the adrenal venous blood led into a measuring cylinder kept in packed ice. The blood was centrifuged and kept deep-frozen for analysis. The plasma was extracted according to the method of Bondy, Abelson, Scheuer, Tseu & Upton (1957). Extracts were developed chromatographically, initially in the system L/75 (Bush, 1952, 1961). The origins of these chromatograms were eluted and re-run in system T/70 (Bush, 1961). One compound was considered to be cortisol on the following grounds: (i) the compound had an RF