Aqueous fluorophotometric, tracer localization and modeling methods were combined to document the existence of a pathway in the normal rabbit for the diffusion of proteins from the ciliary and iridial process stromas through the iris stroma into the aqueous humor of the anterior chamber. A new custom-conjugated tracer, fluoresceinated horseradish peroxidase (F-HRP), was used. Anesthetized rabbits were injected intravenously with F-HRP (250 mg/kg). In some animals, aqueous fluorophotometric and tracer localization studies were performed on the same eyes. Anterior chamber fluorescence was detected 2-10 min post-injection and rose to concentrations of 0.01-0.05 mg/ml 60 min post-injection. Subsequent tracer localization studies of these eyes revealed that the morphologic components of the blood-aqueous barrier were intact, that is, no leakage of F-HRP from the iris vasculature or across the ciliary epithelium was observed. Separate tracer localization studies were performed to examine the time course of the route(s) by which tracer entered the anterior chamber. These studies revealed a "wave" of tracer that migrated from the ciliary and iridial process stromas, through the iris, and arrived at the anterior iris surface approximately 8 min post-injection. A pharmacokinetic model based on the diffusional pathway was developed to describe the time course of the concentration of plasma macromolecules in the ciliary body, iris and anterior chamber. Model predictions were consistent with aqueous fluorophotometric and tracer localization results. The diffusion model can account for a major fraction of protein entering the aqueous humor of normal rabbit eyes.