In man the electroretinogram to pattern reversal stimuli (P-ERG) represents a cone response of the proximal retina, dominated by the cone mechanisms sensitive to red (R) and green (G). Additionally there is a cone mechanism sensitive to blue (B) which can be studied with and without steady exposure to yellow light. During exposure to a superimposed uniform yellow background (576 nm) the transient P-ERG of the B cones is represented by potentials of small amplitude (less than 1 microV). The latency (peak time) of the response is about 30 ms longer than that of the midspectral (R and G) cones. Furthermore, the P-ERG of the B cones saturates at low luminances and exhibits a maximum amplitude at about 460 nm. Without yellow adaptation, the P-ERG of the B cones can be studied only with low-intensity stimuli of short wavelengths. Near threshold, both the long-latency response of the B cones and the short-latency response of the R and G cones are recorded simultaneously, forming a double-peaked wave shape. At suprathreshold luminances, even of short wavelength (435 nm) the P-ERG of the B cones is concealed by the larger short latency response of the midspectral cone mechanism.