12 mg (6 ml) ciprofloxacin were intravenously administered to rabbits. Using high-pressure-liquid-chromatography the concentration in serum, aqueous humor and vitreous were measured after 1, 4, 10 and 24 h. The mean serum levels were 0.698 mg/l and 0.0425 mg/l after 1 and 10 h, respectively. The drug reached mean levels in the aqueous humor of 0.0595 mg/l and 0.0073 mg/l, respectively. Concerning the subconjunctival application 1 mg (0.5 ml) ciprofloxacin was injected either epibulbar near the limbus corneae or under the conjunctiva of the lower fornix. The mean aqueous humor levels were 0.887 mg/l and 0.094 mg/l after 1 and 10 h, respectively, following epibulbar injection. In contrast, the "fornix-injection" produced mean levels of 0.0267 mg/l and 0.025 mg/l in the aqueous humor after 1 and 10 h, respectively. The concentration of ciprofloxacin in the vitreous was near the detection limit following every method of administration of the drug. The importance of differentiating between epibulbar subconjunctival application and the injection under the conjunctiva of the fornix is discussed.