The human cone electroretinogram gradually increases in amplitude an average of 75% (range 23 to 157%) during light adaptation, over a period of approximately 20 min. This increase involves both the a- and b-wave components of this response, and both waves follow a similar time course, implying that the photoreceptors themselves are responsible for the effect. The phenomenon occurs with suprathreshold, but not with threshold, levels of stimulation, and the stronger the test light, the greater the effect. An increase in the intensity of the adapting light shortens the time course of the ERG response, measured as b-wave implicit time, but this occurs almost immediately, and the implicit time then remains constant during the slow increase in response amplitude. The stronger the background adapting light, the smaller is the ERG amplitude, but the percentage growth (or rate of recovery) is unchanged. This slow increase in amplitude is thought to reflect the redepolarization of the cones, after their initial hyperpolarization to an adapting field. It does not reflect the d.c. potential of the eye (the EOG). It is essential to control this phenomenon in any studies of the human cone ERG, in order to minimize variability.