Using a psychophysical method that allows the tracking of very rapid changes in sensitivity, we demonstrate an anomaly in the time-course of light adaptation for the short-wave mechanism: after the onset of a yellow (581 nm) field of approximately 10(5.3) td the threshold for short-wave targets does not recover monotonically but continues to rise for several seconds before falling to its equilibrium value. The phenomenon is absent when the adapting field has a wavelength of 511 nm and has been adjusted to give a similar equilibrium value for the short-wave threshold.