Ocular penetration of 5-fluorocytosine (5FC) was studied in uninfected rabbits after subconjunctival and oral administration. With oral administration, 5FC achieved therapeutic levels in both the vitreous and aqueous humors. The use of a pharmacokinetic model permitted objective comparison of kinetic events within the eye chambers and the serum. The rates of entry and elimination in the vitreous were found to be slower than those in the aqueous, but the mean concentration over 24 hr was the same. The therapeutic levels achieved in the aqueous after subconjunctival administration were of shorter duration, and no detectable levels occurred in the vitreous. Oral administration is clearly therapeutically superior to subconjunctival administration in this model.