The corneal and aqueous humor steroid concentrations produced by topical application of a standard dose of tritiated, micronized 0.125% and 1.0% ophthalmic suspensions of prednisolone acetate were determined. This acetate steroid derivative was able partially to overcome the epithelial barrier of the cornea and, in the face of an intact epithelium, was able to penetrate into the cornea and aqueous humor in high concentration. The 1.0% preparation of prednisolone acetate produced a higher steroid concentration both in the cornea and in the aqueous humor than did the 0.125% preparation under all experimental conditions studied.