We have observed significant variation in absolute dark-adapted thresholds both within and between strains of mice with differing pigmentation levels. Differences in sensitivity within a single strain appear to be related to the Williams' photostasis effect (Balkema, DiLeonardo & Hamdy, SFN, San Deigo, 2001), i.e., photoreceptor rhodopsin levels are dependent upon ambient lighting conditions. In the present study, we maintained albino mice (c2J/c2J) at 10 -4 cd/m2 (dim) and black mice at 103 cd/m2 (bright) in order to equalize their rhodopsin levels. This resulted in ocular rhodopsin levels for albino mice (albino-dim) that were 494 ± 11 pmoles/eye and rhodopsin levels for black mice (black-bright) that were 506 ± 25 pmoles/eye. For comparison, rhodopsin levels in black mice maintained under dim light were 586 ± 46 pmoles/eye and 217 ± 46 pmoles/eye in albino mice maintained in bright light. Absolute dark-adapted thresholds determined with a water maze test showed that albino and black mice with equivalent...