1. The existence and extent of interactions among rods were analysed in the isolated retina of Bufo marinus. Intracellular recordings were made from the outer segments with micropipettes. Stimuli of precisely measured geometry were delivered to the outer segments using a compound microscope. 2. To demonstrate the existence of interactions two rods were simultaneously impaled; current injected into one rod resulted in a current-induced potential of like sign in the other rod. 3. When the diameter of a circular stimulus was increased from 30 to 500 micrometer the mean sensitivity of the rods increased by 1.2 +/- 0.3 log units (S.D.), though the illuminance measured through the impaled rod was adjusted to be constant. 4. The extent of interactions was examined by presenting a dim, slit-shaped stimulus at each of several displacements from an impaled rod. This stimulus either passed through the retina before coming into focus on the outer segments (transillumination) or was focused directly on the outer segments (incident illumination). For each displacement both the amount of light scattered onto the impaled rod and the response of the rod were measured. 5. The amount of light scattered onto the impaled rod was assessed for each displacement of the stimulus. For incident illumination this assessment was made by measuring the distribution of bleached pigment about the slit stimulus. The bleaching could be described by an exponential that decreased with distance; this exponential had a space constant of 4 micrometer. For transillumination the assessment was made in two ways. The light that passed through the impaled rod was measured and found to decrease exponentially with slit displacement; the mean space constant of this exponentially was 9.3 +/- 3.2 micrometer (S.D.). In few experiments the distribution of bleached pigment about the slit stimulus was measured and was found to decrease exponentially with distance; this latter exponential had a slightly longer space constant than the exponential measured using transmitted light. 6. For each impaled rod a convolution of an exponential interaction function with the measured distribution of stimulus illuminance about the impaled rod could be fitted to the plot of response amplitude versus displacement of the slit stimulus. For stimuli presented by transillumination the mean space constant describing the interaction functions was 20 +/- 6 micrometer (S.D.). For stimuli presented by incident illumination the mean space constant of the interaction functions was 22 +/- 4 micrometer (S.D.). 7. This report presents new evidence that interactions among rods exist in the isolated retina and that the extent of interactions can be described by a space constant of about 20 micrometer.
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