60 rats were divided into one control and 2 test groups of 20 male animals in each. Brains from control animals were taken out immediately after killing, and in the test groups (I and II) after brain cell damage by controlled autolysis at 37 degrees C for 6 and 18 h, respectively. Ten animal brains from each group were used for estimation of RNA content. The remaining 10 brains each of the test and control groups were homogenised in 0.001% thionin in formaline saline and incubated for thionin binding. The supernatants were read spectrophotometrically and the amount of thionin in the sediment(s) was calculated. The amount of sediment bound thionin in mg per 100 g brain was expressed as "thionin binding capacity" (TBC) of the brain. RNA content of the brain in control and test groups (I and II) was found to be 248.33 +/- 6.83 (100%), 210.83 +/- 8.61 (84.9%), and 190.83 +/- 10.21 (76.85%) mg per 100 g of the brain. Thionin binding capacity was 6.30 +/- 0.12 (100%), 4.82 +/- 0.17 (76.45%), and 3.49 +/- 0.39 (55.45%) mg per 100 g brain, in the 3 groups respectively. Both parameters showed similar decreasing tendencies with increasing brain damage i.e. in the groups I and II. The decrements of the 2 parameters were highly significant (P less than .001), hence, they could be utilised to assess the brain damage.