The retina of the Royal College of Surgeons (RCS) strain of rat, which is being used as an animal model for human retinal degenerations, has been employed in the study of the function of second order neurons. By about the 33rd postnatal day the dendritic branching of isolated bipolar cells is more sparse than in bipolar cells of the normal rat retina, but their GABA channels are as in the normal rat retina. The normally occurring light-induced distal potassium increase has been used as the indicator of the functional competence of second order neurons in the isolated RCS rat retina. These are dependent upon the integrity of ionotropic and metabotropic synapses. At about the 22nd postnatal day MgCl2 enlarges the light-induced distal potassium increase in the young RCS rat retina as in the normal rat retina. It seems that MgCl2 does not block the metabotropic synapses of on-bipolar cells. At about postnatal day 33, at which time the photoreceptors of the RCS rat retina had become severely damaged, the size of this light-induced distal potassium increase was not changed, but it was abolished by MgCl2. This indicates that bipolar cells are still active but that the synaptic function of on-bipolar cells has become vulnerable to MgCl2. The conclusion is that at a time when photoreceptor degeneration is already severe bipolar cells are still active, but that on-bipolars, mainly rod bipolar cells, have some functional deficit.