The movement of carboxyfluorescein across the isolated iris-ciliary body of the albino rabbit was analyzed using an Ussing chamber under short-circuit conditions with carboxyfluorescein concentrations ranging from 10 to 100 microM. At a concentration of 50 microM, the outward permeability (from the aqueous to the stromal side) was 4.0 +/- 0.2 x 10(-6) cm/sec and the inward permeability (from the stromal to the aqueous side) was 1.3 +/- 0.2 x 10(-6) cm/sec (Mean +/- SE, N = 6). The former was significantly greater than the latter (P < 0.001). Inward movement of carboxyfluorescein showed a linear increase in relation to the concentrations tested. Neither ouabain (100 microM), 2,4-dinitrophenol (10 microM) nor probenecid (100 microM) significantly inhibited the inward movement of carboxyfluorescein. The net outward movement of carboxy-fluorescein became saturated, as concentration was increased, and a Lineweaver--Burke plot gave an apparent Km of 28 microM and Vmax of 0.67 x 10(-9) mole/hr/cm2. In addition, outward carboxyfluorescein movement was significantly inhibited by ouabain (100 microM), 2,4-dinitrophenol (10 microM), probenecid (100 microM), iodipamide (1.0 mM), hippurate (1.0 mM), low temperature or low external Na+ concentration. These results suggest that outward carboxyfluorescein movement across the tissue largely depends on carrier-mediated active transport, while inward movement occurs by passive diffusion.