Isolated bovine cornea was found to reduce the 20 ketone group in fluorometholone (FML) to produce a mixture of 20 alpha- and 20 beta-dihydrofluorometholone (DHFML). Incubation of FML with bovine cornea at pH 7.4 in phosphate buffer produced an increase in the amount of reduction products up to 8 h; after this time the rate of reduction decreased. The reduced products were quantified by GC-MS against [2H3] 20 alpha-DHFML. Variable amounts of 20 beta-DHFML were detected in six samples of human aqueous humour following topical application of 50 micrograms of FML to the eye. The rapid metabolism of the side chain of FML may account for the unexpected low concentrations of FML which are found in human aqueous humour following its application to the surface of the eye.