In the retina of rat, cones make up approximately 0.85% of the photoreceptor population: 93% of these cones contain a midwave-sensitive pigment, the rest expresses a short-wave-sensitive pigment (Szel & Rohlich, 1992). We used normal adult Long Evans rats to determine the spectral sensitivity of the cone-driven electroretinogram (ERG) b-wave and its absolute sensitivity at lambda(max) of the cone pigments. ERGs were recorded at the cornea of anesthetized animals under dark- and light-adapted conditions. Rod responses were suppressed by steady rod-saturating orange backgrounds and/or by a flashed "white" background. Cone-driven b-waves were evoked by "white" or narrowband full-field stimuli of varying intensity. The action spectrum for the cone b-wave indicates the presence of an absorbance peak at 510 nm; a second, twofold lower, peak was found at 360 nm (after correction for transmittance by the lens). Chromatic adaptation experiments strongly suggest that retinal responses to midwave and UV stimuli are mediated by a single cone type. On a background producing approximately 17,000 R* rod(-1) s(-1), which completely suppressed the saturated a-wave, the absolute sensitivity of the cone b-wave was 18 nV photon(-1) microm2 at 510 nm and 4 nV photon(-1) microm2 at 360 nm which is 20-30 times higher than for the mouse. It is suggested that the relatively large number of on-cone bipolar cells in the retina of rat is responsible for the remarkable sensitivity of the cone b-wave.