IOP is increased during the dark compared to light periods in several strains. Mean IOP ± SEM is shown for the indicated strains at measurement during the light (open bars) or dark (filled bars) periods of the day. All mice were 2 to 4 months old except for CBA/CaJ that were 5 to 6 months old. 129B6 refers to mice of a mixed 129X1/SvJ and C57BL/6J background. IOP was increased during the dark in all strains except CBA/CaJ. The number of mice successfully analyzed during the light (L) or dark (D) periods follows, DBA/2J 10L,10D; C57BL/6J 32L, 22D; C57BR/cdJ 12L,10D; SWR/J 16L, 16D; BALB/cByJ 15L, 16D; 129B6 10L, 12D; CBA/CaJ 22L, 16D.