SUMMARY1. Fresh bovine, pigmented ciliary epithelial cells possess an inward rectifier current activated by hyperpolarization. This current was investigated using wholecell patch-clamp techniques. At the holding potential of -70 mV, and with EK (potassium equilibrium potential) set at -84 mV, a small outward current flowed through the inward rectifier that was sensitive to external K+, becoming more outward in 0 5 mm K+ and progressively more inward in 20 and 50 mm K+.2. The inward rectifier showed V-EK dependence; increasing [K+]o increased the inward conductance from 1-28 nS in 5 mm K+ to 7-42 nS in 50 mm K+. The conductance at a given V-EK was proportional to the square root of [K+]0.3. It was blocked by external Cs+ but replacing K+ in the pipette with Cs+ blocked only outward ion movement through the inward rectifier. Inward rectification was also blocked by Ba2' (85% with Ki (concentration giving half-maximal inhibition) -31 x 10-5 M) and TEA+ (74% with Ki = 2-9 x 10-4 M).4. The activation time constant was voltage dependent, decreasing from 5 ms to 0 7 ms over the voltage range -90 to -170 mV. With increasing hyperpolarization the current exhibited time-dependent decay. The time constant for this process was voltage sensitive but the steady-state inactivation was independent of external [K+].5. The current disappeared in culture within 8 days. 6. Solution flow over the cell inactivated the inward rectifier, a property that may be related to [K+]0.7. In current clamp the cells exhibited an unstable region at a potential of around -70 mV. Once in this region oscillations and regenerative hyperpolarizing potentials were observed. This behaviour was eliminated by treatments that blocked (Cs+, Ba2+) or removed (0 5 mm K+) active inward rectification.8. It is suggested that these oscillations may represent a process of cation loading, the first step in the secretion of aqueous humour.