Membrane voltage was recorded in rod photoreceptors in retina isolated from macaque monkey. The size of the single photon response and the magnitude of membrane voltage fluctuations were assessed in dark- and light-adapted retina. The "dark light" rate I(D), defined as the rate of spontaneous photopigment isomerizations that would produce a variance equivalent to that of the noise measured in the dark, was calculated after matched filtering. The average value of 0.08 s(-1) fell at the higher end of psychophysical estimates of dark light in human observers. In light-adapted rods the photon response decreased in amplitude and duration, and the magnitude of the voltage fluctuations increased with increasing background light intensity. The signal-to-noise ratio (SNR) for single rods was defined as the ratio of the peak amplitude of the photon response to the standard deviation of the noise fluctuations. The signal-to-noise ratio for dark-adapted rods SNR(D) was about 7. With increasing background intensity I, the SNR fell as SNR(D)(1 + I/I(D))(-1/2). This function may account for the increment thresholds measured with small brief test flashes in human psychophysical experiments.