The goal of these experiments was to define the time course and degree of cone adaptation in primate outer retina by use of probe stimuli upon temporally modulated backgrounds. Recordings were obtained from primate horizontal cells. Test probes were either a low-amplitude, high-frequency sinusoid superimposed on a slowly modulated background or small test pulses superimposed on backgrounds of various frequencies. The amplitude of the test response was modulated by the background, indicating sensitivity regulation. Results were consistent with gain controls which, at 1000 td, required approximately 10-20 ms to completion. These mechanisms could also account for some of the distortions of horizontal cell responses to sinusoids and pulses. Modulation of test responsivity occurred at low background contrasts, suggesting no threshold change in light level must be exceeded to evoke sensitivity regulation. As retinal illuminance increased from darkness, sensitivity regulation was evident at 10-20 td.