The tight junctions of the human ciliary epithelium have been studied using the freeze-fracture technique with particular regard to regional differences in the tight junction networks and implications on the transepithelial resistance. The tight junctions of the non-pigmented epithelium always form continuous networks and consist of from two to more than 20 superimposed strands (mean 3 to 4), suggesting that relatively tight and leaky sites of the paracellular pathway coexist within the same cell perimeter. The tight junction morphology is more complex in the anterior pars plicata than in the posterior pars plicata, but its complexity increases again towards the pars plana. The application of an electrical circuit analysis to morphometric freeze-fracture data suggests that the transepithelial resistance decreases from the anterior pars plicata (32 omega cm2) towards the posterior plicata (26 omega cm2) and becomes lowest in the pars plana (19 omega cm2) and that the transepithelial resistance of the native epithelium is lower than most reported calculations based on electrophysiological measurements. We conclude that human ciliary epithelium is a leaky layer, but that the anterior ciliary processes are less leaky than the posterior processes and the pars plana despite the occurrence of the breakdown of the blood-aqueous barrier predominantly in the anterior pars plicata.