Growth and survival studies were conducted in adrenalectomized immature male rats following single subcutaneous injections of aqueous suspensions of a series of 9[alpha] -halo derivativesof hydrocortisone acetate. 9[alpha] -Chloro-hydro -cortisone acetate is 10-20 times and 9a -bromo-hydrocortisone acetate 2.5 times as active as desoxycorticosterone acetate (DCA). 9a-Fluoro-hydrocortisone acetate, hydrocortisone acetate, and cortisone acetate are less effective than DCA. Na retention assays carried out in adrenalectomized male rats after administration of a Na22 load indicate that 9a-chloro-hydrocortisone acetate is 10.8 times as potent a Na-retainer as DCA, that 9a-fluoro-hydrocortisone acetate is also somewhat more effective than the latter compound and 9a-bromo-hydrocortisone acetate considerably less effective. Dose-response data suggest that the steroids cause Na retention by more than 1 mechanism.