To investigate mechanisms underlying sensitivity changes that are capable of following rapid variations in intensity of the background field, we measured the threshold radiance needed to detect a 2-ms probe flash presented at various phases relative to a sinusoidally flickering background. The temporal frequency, mean luminance, and modulation of the background were systematically varied. The sensitivity change consisted of two components: a phase-insensitive increase in threshold that occurs at all the phases of the background field (a change in the dc level of the threshold), and a phase-dependent variation in threshold. Both components can reliably be measured at temporal frequencies up to approximately 50 Hz. On a 30-Hz background, the threshold varied with phase over roughly 0.5 log unit within a half-cycle (17 ms). For background flicker rates of 20-40 Hz the probe threshold increased with increasing instantaneous background radiance, following a typical threshold-versus-radiance template, and approaching Weber-law behavior during the peak of the background flicker. This pattern of threshold elevation was measured at mean background illuminances from 580 to 9100 Td (trolands), with the dimmer backgrounds being slightly less effective in producing threshold elevations. The measured increase in the dc level commenced as soon as the modulation of the background flicker began, and the amount of threshold elevation followed the envelope of the background flicker, ruling out modulation gain control explanations for the change in sensitivity on flickering backgrounds. The threshold elevations measured on a 30-Hz, 25% modulation background were lower than those measured on a 30-Hz, 100% modulation background at all phases. The measured changes in threshold with changes in background modulation rule out all adaptation models consisting of a multiplicative and a subtractive adaptation processes followed by a single, late, static nonlinearity.