Polyacrylamide gel electrophoresis of the H1 group of histones extracted from different rat and mouse tissues shows a different pattern of fractions (HI, H1°, H1° met) when the two species are compared. Different tissues of each species also have a different pattern of HI histone fraction and subfraction. However, the age-associated changes of mouse HI histones from liver and spleen chromatin show the same type of alteration of fraction ratios which had been demonstrated in our earlier research with rat tissues. In both species there is a relative increase of the F1° and F1° met fractions in tissues from old animals. The presence of the F1° fraction in only non-dividing cells suggests that there may be an age-specific type of chromatin condensation.