SUMMARY: The objective in the present study was to develop a combined dexamethasone-suppression cosyntropin-(synthetic ACTH-) stimulation test (cdc test) in horses so that information concerning pituitary gland (hypophysis) and adrenal gland competence could be obtained in one relatively short test procedure. In the combined test, time-response relationships between the plasma hydrocortisone (cortisol) concentrations and the dexamethasone and cosyntropin injections were determined. The plasma hydrocortisone concentration (measured by competitive protein-binding [cpb]) decreased to 27% (P < 0.05) of base-line values 3 hours after dexamethasone (20 mg, im) was injected. Further suppression of hydrocortisone concentration did not occur at 5 hours. After cosyntropin was administered (1.0 mg or 100 IU, iv), maximal plasma hydrocortisone concentration (187% of base line) occurred at 2 hours, and the value at 4 hours approached base line (119% of base line). Mean plasma hydrocortisone concentration further declined to suppressed values (18% of base line) 19 hours after cosyntropin treatment, indicating that dexamethasone was still suppressing hydrocortisone release.