We have investigated the adrenal response, in eight healthy adult men, to low doses of ACTH(1–24) in order to define a dose which will elicit a response similar to that obtained with the short Synacthen test. The studies were performed at 14.00 h and blood samples were withdrawn at 5-min intervals after an i.v. bolus injection of ACTH(1–24). The sampling interval was crucial in determining the timing of the peak response. Using sampling times of 0, 10, 15, 20, 25 and 30 min ensured detection of 47 out of 48 peaks. A dose-dependent rise in plasma cortisol concentration was observed with bolus injections of ACTH(1–24) between 30 and 250 ng/1·73 m2 body surface area. Increasing the dose to 500 ng/1 ·73 m2 (500 times less than that used in the short Synacthen test) elicited an increment of plasma cortisol concentration of 200 nmol/l or greater in all subjects. Pretreatment with dexamethasone (1 mg) did not alter the timing of the peak cortisol concentration but blunted the increase (pretreatment: median, 159 nmol/l; range 83–239; on dexamethasone; median 62 nmol/l; range 21–207; P = 0·04). These data suggest that a dose of ACTH(1–24) of 500 ng/1·73 m2 satisfies the criteria of the short Synacthen test and may provide a useful method of investigating adrenal function. Journal of Endocrinology (1991) 130, 475–479