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1. Intracellular responses were recorded from rods in isolated eye-cups of the snapping turtle. Chelydra serpentina. Responses to flashes of small (less than 100 mum diameter) and large (1000 mum diameter) spots of 500 nm light were studied. 2. Responses produced by small and large diameter spots which delivered less than 0-3 photons mum-2 had the same shape. The responses produced by large spots were, however, nearly ten times greater in amplitude. The difference in amplitude is termed enhancement. 3. Perfusing an eye-cup with a Co2+-containing medium blocked synaptic transmission from receptors to horizontal cells but did not affect the responses of rods. 4. The membrane conductance of a single rod, estimated by three independent methods, was approximately 1-2 X 10(-9) MHo. 5. Enhancement can be predicted by a mathematical model which treats rods as an electrical syncytium. The space coefficient describing the spread of current is approximately 65 mum indicating that the coupling conductance between rods was relatively high. 6. When the intensity of a small spot was increased from 0-3 photons mum-2 up to 6 photons mum-2, the shape of the response was unchanged. When the intensity of a large spot was increased to more than 0-3 photons mum-2, the voltage during the recovery phase was decreased. This decrease is termed disenhancement. 7. The voltages produced by bright, large and small diameter spots which delivered the same quantity of light to the impaled rod were compared. The voltage produced by a large diameter spot became for a short period during the recovery phase less than the voltage produced by a small diameter spot. This observation indicates that the response to a large spot included during recovery an active process which is not apparent in the response to a small spot.

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