To describe the pharmacokinetics of 0.5 mg of intravitreal ranibizumab (Lucentis) and to compare it with that of 1.25 mg of intravitreal bevacizumab (Avastin), using the same rabbit model. Experimental animal study. Twenty-eight Dutch-belted rabbits. One eye of each of 20 rabbits was injected with 0.5 mg of intravitreal ranibizumab. Both eyes of each of 4 rabbits were enucleated at days 1, 3, 8, 15, and 29. Ranibizumab concentrations were measured in aqueous fluid, whole vitreous, and serum. A further 8 rabbits were used to measure serum and fellow ranibizumab at additional time points of 3 and 8 hours. Ranibizumab concentrations in the aqueous, vitreous, and serum. Although vitreous concentrations of ranibizumab declined in a monoexponential fashion with a half-life of 2.88 days, concentrations of >0.1 microg/ml ranibizumab were maintained in the vitreous humor for 29 days. Ranibizumab concentrations in the aqueous humor of the injected eye reached a peak concentration of 17.9 microg/ml, 3 days after drug administration. Elimination of ranibizumab from the aqueous humor paralleled that found in the vitreous humor, with a half-life value of 2.84 days. No ranibizumab was detected in the serum or the fellow eye. In the rabbit, the vitreous half-life of 0.5-mg intravitreal ranibizumab is 2.88 days, shorter than the half-life of 1.25-mg intravitreal bevacizumab of 4.32 days. No ranibizumab was detected in the serum or the fellow uninjected eye; whereas small amounts of intravitreal bevacizumab have been detected in the serum and fellow uninjected eye.