The permeability of the blood-ocular barrier was investigated in five monkeys using vitreous fluorophotometry (VFP). Inward permeability (Pin) of the blood-retinal barrier was calculated by a computer simulation method. Kinetic VFP was performed after intravitreal injection of fluorescein (F) or fluorescein monoglucuronide (FG). The estimated mean value of Pin (x10(-6) cm/min) was 4.8 (SD 1.2). The mean rates of loss (per hour) of F from the anterior chamber (Ka) and the vitreous (Kv) were 0.11 (SD 0.01) and 0.13 (SD 0.03), respectively, which were approximately three and four times greater than those of FG (0.04 (SD 0.01) and 0.03 (SD 0.01), respectively). Probenecid administered intraperitoneally decreased both the Ka and the Kv of F significantly but had no effect on the Ka or the Kv of FG, suggesting that F was excreted from the eye with the aid of the active transport mechanism. The results of comparative studies of the rates of loss of F from the anterior chamber (Ka) and from the vitreous (Kv) suggested that active transport was more predominant in the blood-retinal barrier than in the blood-aqueous barrier.