The short wavelength sensitive (S) cone electroretinograms (ERGs) were examined in two patients with Oguchi's disease to study S cone function. Ganzfeld colour flashes under bright white background illumination were used to elicit S cone, and mixed long (L) and middle (M) wavelength sensitive cone ERGs. The S cone ERG b-wave was normal with short wavelength stimuli with normal L and M cone responses in both patients. These ERG results indicate that the S cone system as well as the L and M cone system is not defective in Oguchi's disease, while the S cone responses are not detectable in the complete type of congenital stationary night blindness as reported previously. The results imply that there is a significant difference in the way the S cone system is affected in different forms of stationary night blindness, and that the S cone and rod do not share the same arrestin system.