Carp H2 horizontal cells were investigated at various stages of dark and light adaptation. Their state of adaptation was verified by evaluating in computer reconstructed serial sections the number of small, fingerlike protrusions originating from H1 processes. These so-called spinules are absent in the dark-adapted state and are reformed during light adaptation. Intracellular recordings showed that long-wave-induced depolarization thought to be mediated by sign-inverted feedback from H1 horizontal cells onto green-sensitive cones could not be demonstrated in completely dark-adapted specimens. This type of response only developed with increasing light adaptation. Following electrophysiological characterization, the cells were injected with horseradish peroxidase and identified light and electronmicroscopically. Our results show a good correlation between the number of spinules and the degree of electrophysiological feedback suggesting that the spinules may be the sites of the feedback synapses in teleost retinas. Several models of synaptic transmission from H1 terminals to cone pedicles are discussed.