1. Small responses to large fields of dim monochromatic lights were recorded intracellularly from luminosity horizontal cells (L-cells), chromaticity horizontal cells (C-cells) and cones in the retinae of turtles, Pseudemys scripta elegans.2. Responses of cones to brief flashes applied over steady backgrounds were studied in order to interpret the corresponding responses of horizontal cells. Steady red or green backgrounds make the responses of red-sensitive cones smaller, faster and often diphasic. Green backgrounds have similar effects on the responses of green-sensitive cones to green flashes, but red backgrounds do not change them appreciably. Responses of double cones have properties intermediate between those of red and green cones.3. L-cells of both type I and type II are hyperpolarized by all visible wave-lengths, and their spectral sensitivity in the linear range resembles that of red cones. Their responses are not invariant with respect to colour, and their sensitivity to green relative to red stimuli increases during red backgrounds. These properties suggest that L-cells are activated mainly by red cones but also receive impingement from the red members of double cones.4. Spectral properties of red/green C-cells resemble those of green cones as modified by the recurrent action of L-cells. They can be explained assuming that red/green C-cells receive their principal impingement from green cones and subsidiary interactions from green/blue C-cells and the green members of double comes.5. The spectral sensitivity of the hyperpolarizing responses of green/blue C-cells is ascribed to impingement from blue cones. Their depolarizing responses have complex properties which suggest that they are brought about by the activity of both L-cells (probably through the blue cones) and red/green C-cells.6. It is concluded that the main properties of the responses of the horizontal cells can be explained by a simple circuit in which each horizontal cell is connected to a corresponding type of cone and the L-cells have a recurrent impingement on all cones. The scheme is modified by additional interactions which operate on the responses of each horizontal cell type.