||Thin-layer chromatography, acetylation of reference and unidentified glucocorticosteroids, u.v. absorption and fluorescence induced in sulphuric acid were used to identify cortisol as the major free plasma glucocorticosteroid in the horse (Equus caballus), with cortisone and corticosterone as minor glucocorticosteroids. Deoxycorticosterone was also identified. The plasma ratio for free cortisol: cortisone: corticosterone was 16:8:0·5.The diurnal variation was determined for all three glucocorticosteroids. The highest levels of cortisol and corticosterone were found at 10.00 hr. (260 and 10·3 μg./100 ml., respectively) and the lowest concentration of cortisol at 02.00 hr. The highest level of cortisone occurred at 02.00 hr. (140 μg./100 ml.), and the lowest appeared at 16.00 hr. (65 μg./100 ml.). The maximum plasma concentration of the combined glucocorticosteroids was found at 10.00 hr. (395·3 μg./100 ml.), and the minimum at 20.00 hr. (219 μg./100 ml.).