The mammalian neural retina is weakly attached via the retinal pigment epithelium (RPE) and choroid to the back wall of the eye, and its detachment results in serious impairment of vision. Complementary surfaces of photoreceptors and RPE are believed to be kept apposed by (i) absorptive, outward transport of water by the RPE and (ii) bridges of cell adhesion molecules like αVβ5 integrins (1). We explored the possibility of enhancement of spontaneous water absorption by the RPE as an attractive strategy for reattaching the retina. Bovine eyes were obtained as abattoir material (Deonar Abattoir, Mumbai) from male oxen (B. taurus), aged between 12 and 14 years. Circular pieces of RPE and adherent choroid were mounted initially in supplemented Ringer solutions in Kel-F Ussing chambers, either vertically for high-resolution (0.01% volume change detection) measurement of transepithelial water flow (2) using a capacitance probe (epithelial area 2.0 cm2, 32 ± 1oC) or horizontally (area 0.78cm2, 30 ± 1oC) for measurement of transepithelial electrical parameters with microelectrodes. Data are presented as mean ± SEM and were compared for statistical significance using paired two-tailed Student’s t-test. Each n value for water flow measurement is a moving average of a minimum of 1500 digitized data points acquired once a second. Our salient observation is the stimulation (sustained for >5 hrs) of absorptive water flow by the RPE from 0.2 ± 0.3 to 4.8 ± 0.4μl.cm-2.hr-1 (n=16; p<0.001) in symmetrical Ringer solutions by 1 mM HgCl2 applied to the retinal side. HgCl2 doubled the passive osmotic (sucrose) water permeability from 126 ± 19 to 254 ± 54 μl.cm-2.hr-1.mol-1 (n=7; p<0.05). The retina positive transepithelial potential decreased from 5.5 ± 0.3 to 0.3 ± 0.04 mV and transepithelial resistance decreased from 352 ± 12 to 42 ± 3 Ω.cm2 (n=43, p<0.001). The augmentation of water absorption by HgCl2 was sustained for hours, without cell death (as inferred from hyperpolarized membrane potentials), or epithelial disruption (as inferred from electron microscopy) and was dependent on the presence of both Na and Cl ions. The RPE is clearly a macrophage system that can perform solute and water transport functions and remain viable in the presence of heavy metals. The data reveal the existence in the RPE of latent NaCl transport mechanisms that can be recruited to generate sustained and powerful water absorption, in the correct direction, to therapeutically appose the retina to the RPE in vivo.