Two genetically engineered strains of mice were used to characterize murine cone function electroretinographically, without interference of rod-driven responses: (1) mice with a deletion of the gene for the rod transducin alpha-subunit (transducin alpha-/-), and (2) mice with rod arrestin deleted (arrestin -/-). In the first three months of age, both strains have a normal complement of rods and normal rod structure, but transducin alpha-/- mice have no rod-driven responses to light, while rod-driven activity of arrestin -/- mice can be suppressed by a single intense flash for hours. In response to intense flashes the electroretinograms of these strains of mice showed a readily identifiable, pure-cone a-wave of approximately 10 microV saturating amplitude. A 530 nm background that saturates rod responses of wild type mice was found to desensitize the b-wave responses of mice of both transgenic lines, whether the b-waves were driven by photons captured by M- or UV-cone pigments. The desensitizing effect of the 530 nm background on UV-pigment driven responses provides new evidence in support of the hypothesis of functional co-expression of the M-pigment in cones expressing primarily the UV-pigment.