We describe a new response in proximal retina of cat that is present under scotopic conditions, clearly differs from PII (b-wave and DC component) and contributes a negative potential at threshold to the diffuse electroretinogram (ERG). We have termed this response the scotopic threshold response (STR). Extracellular potentials evoked in response to circular spots of light at dark-adapted threshold, and with dim backgrounds, were recorded with microelectrodes placed intraretinally at different depths. The dark-adapted response of proximal retina (STR) consisted of a graded negative potential to the onset of illumination that maintained amplitude during illumination and decayed back toward the base line at stimulus offset without evidence of a negative-going off response. It thereby differed in form from the photopic M-wave response of proximal retina, which has a negative-going off response. It also did not exhibit spatial tuning, simply increasing in size with stimulus area. In addition, the STR appears to be a rod-driven response whose threshold approximates that of the most sensitive ganglion cells in cat, whereas the M-wave is a much higher threshold cone-driven response. The STR could be clearly distinguished from PII on the basis of its form, depth-distribution, and dynamic range. For example, the STR had its maximum amplitude in the proximal retina at 17% retinal depth, whereas scotopic PII had its maximum in the distal retina at 48% retinal depth. Also, the STR had a lower threshold than PII intraretinally and saturated well below the level of saturation of scotopic PII (rod saturation). By analogy to the PNR and M-wave, the STR is hypothesized to represent either an extracellular voltage arising from proximal retinal neurons or Müller cell responses to K+ released by these neurons. Recording in the vitreous, near the retinal surface, showed that the STR always had a negative polarity. The polarity reversal of the STR at 50-60% retinal depth (from negative, proximal to positive, distal) suggested the presence of a sink proximal to the reversal point and a source distal to it. We also recorded the vitreal ERG in response to diffuse illumination of the dark-adapted retina. The STR could be clearly identified in the scotopic ERG as a threshold negative potential that had been observed previously in the mammalian ERG. The STR differs, therefore, from PII (b-wave and DC component) that is a higher threshold positive component in the diffuse ERG.(ABSTRACT TRUNCATED AT 400 WORDS)