The standard intravenous short Synacthen test (SSST) has long been accepted as one of the most reliable diagnostic tests of adrenocortical insufficiency. Intramuscular (i.m.) administration of ACTH obviates the need of venous cannulation and can be used as an alternative to the intravenous test. Nevertheless, reports of correlation between cortisol response to i.m. ACTH1-24 and 24-hr average cortisol concentration are scarce. We studied this relation in 64 nonobese healthy men. Blood samples for serial cortisol measurements were collected hourly over 24 hrs. The following day, blood samples were collected at baseline and at 30 and 60 min after intramuscular (i.m.) administration of 250 microg of ACTH1-24. All healthy men reached 24-hr serum cortisol peak values (Cmax) between 0600 h and 1000 h. Following i.m. ACTH1-24, cortisol levels significantly increased at both 30 (C30ACTH) and 60 (C60ACTH) minutes, when compared to baseline values. C30ACTH and C60ACTH significantly correlated with Cmax and with the 24-hr time-integrated cortisol concentration (AUC0-24). Morning mean cortisol was calculated as the average of serum concentrations measured between 0600 h and 1000 h (C(av)6-10) and correlated very well with AUC0-24. In conclusion, we confirmed that i.m. administration of ACTH1-24, followed by a single blood sampling at 60 min for cortisol measurement represents a valid, convenient and cost- effective screening test of adrenal function.