Illumination of GH₃ cells in the presence of uroporphyrin produced an increase in membrane permeability measured by patch clamp electrophysiology and designated I_LEAK. The production of I_LEAK was shown to be dependent on uroporphyrin concentration, intensity of illumination and duration of illumination.  Reciprocity between illumination intensity and duration was shown to hold over a restricted range, such that the amount of I_LEAK was dependent, to a first approximation, on the total number of incident photons. Either ascorbate (≥0.1 mM) or butylated hydroxytoluene (BHT) (≥2 µM) was capable of inhibiting the production of  I_LEAK.  The reversal potential of I_LEAK was about –10 mV before illumination and became more positive during illumination, reaching a plateau near 0 mV or slightly positive to 0 mV. BHT or ascorbate did not significantly alter the change in reversal potential, although a small effect of these antioxidants on the value before illumination cannot be excluded. The current-voltage plots of I_LEAK were linear. This linearity and the reversal potential data suggest that I_LEAK flows through a relatively nonselective ionic pathway.