By extracellular recording of action potentials from single ganglion cells in the dark-adapted eyecup of the frog, we show that four different characteristics of a cell, the frequency-of-response function, the variability in responses to low suprathreshold stimulation, the maintained discharge, and the susceptibility to desensitization by weak background lights, all seem to reflect the same intrinsic noise. In a small fraction of reliable cells this noise is close to that expected from dark isomerizationlike events in rods, but it differs by orders of magnitude between cells. We have also determined a threshold intensity for the phototactic jumping behavior of frogs in darkness. This intensity, while below the human threshold, is still above the threshold for an appreciable fraction of frog retinal ganglion cells, and the sensitivity of this visually guided behavior at 16 degrees C is not strictly limited by dark events in rods.