IT has long been known that vertebrate photoreceptors can signal the absorption of single quanta of light1. The extraordinary sensitivity of the visual system is usually ascribed both to the sensitivity of the photoreceptors and to the integration of their signals by higher-order neurones. A more complete explanation of visual sensitivity could be given if it were possible to record the response of receptors at the visual threshold. Unfortunately, it has not been possible to do this at the absolute threshold, largely because of interactions between receptors2. Near the absolute threshold only a small proportion of the receptors absorb quanta, but the photocurrent from these cells spreads through electronic synapses3 to many other cells. Thus the voltage responses in the receptors are non-uniform, and it is not possible to tell which responses contribute to the threshold. Fortunately this difficulty does not arise at the increment threshold since in the presence of bright background light, the variation in response from one receptor to the next is minimal. The differences among the receptors in the number of quanta caught will be much less important than at absolute threshold, and the connections between the receptors will tend to equalise their voltages. In this report we compare receptor responses to behavioural responses in the light-adapted turtle and show that the receptor response at threshold is only a few microvolts in amplitude.