The duration of suppression of the pituitaryadrenal axis after single injections of three commonly used derivatives of 9-α-fluoroprednisolone was studied. Triamcinolone acetonide (TCA) in 40 mg doses decreased corticosteroid levels for as long as four weeks after injection, whereas 50 mg of triamcinolone diacetate (TCDA) and 9 mg of betamethasone acetate-phosphate mixture (BAP) had adrenal suppressive effects persisting for only one week. This correlates, to a great degree, with the duration of the therapeutic effect observed with each of these preparations. The data indicate that in selecting a depot corticosteroid preparation for intramuscular administration, it is essential to keep in mind the relatively prolonged adrenal suppression that occurs with TCA as compared to TCDA and BAP