Previous work has shown that the cat retinal pigment epithelium (RPE) is the source of two potential changes that follow the absorption of light by photoreceptors: a hyperpolarization of the apical membrane, peaking in 2-4 s, which leads to the RPE component of the electroretinogram (ERG) c-wave, and a depolarization of the basal membrane, peaking in 5 min, which leads to the light peak. This paper describes a new basal membrane response of intermediate time course, called the delayed basal hyperpolarization. Isolation of this response from other RPE potentials showed that with maintained illumination the hyperpolarization begins approximately 2 s after light onset, peaks in 20 s, and slowly ends as the membrane repolarizes over the next 60 s. The delayed basal hyperpolarization is very small for stimuli less than 4 s in duration and grows with duration, becoming approximately 15% as large as the preceding apical hyperpolarization with stimuli longer than 20 s. Extracellularly, this response contributes to the transepithelial potential (TEP) across the RPE. In response to light the TEP first rises to a peak, the c-wave, as the apical membrane hyperpolarizes. For stimuli longer than approximately 4 s, the decline of the TEP from the peak of the c-wave results partly from the recovery of apical membrane potential and partly from the delayed basal hyperpolarization. For long periods of illumination (300 s) the delayed basal hyperpolarization leads to a trough in the TEP between the c-wave and light peak. This trough is largely responsible for a corresponding trough in vitreal recordings, which has been called the "fast oscillation." The term "fast oscillation" has also been used to denote the sequence of potential changes resulting from repeated stimuli approximately 1 min in duration. In addition to the delayed basal hyperpolarization, such responses also contain a basal off-response, a delayed depolarization.