This study was designed to observe the effect of ethanol extracts from fish flour on the nucleic acid metabolism in rats. Young rats, weighing 75-85g were used as the experimental animals and diet used were 8 kinds; diet supplemented with 10% fish flour, diets which were supplemented with the extracts and or remainders of fish flour after extracting by either 76% or 96% ethanol to the rice diet, respectively, and diet supplemented with 6% casein. After feeding corresponding diet for 40 days, RNA and DNA contents, and DNase activities in the liver, kidney and braid were determined. The results obtaioed from this study are summarized as follows: 1. The RNA contents of the ethanol-treatment groups are, in the liver and kidney, similar to, and in the brain, generally higher than, that of the control group. 2. The DNA contents of each organ show no difference between ethanol-treatment groups and control group, but in the liver, of ethanol extrat groups are lower than casein group. 3. the DNase activity of each organ in the ethanol-treatmeut groups, is generally lower than the control group. The above results indicate that ethanol extracts from fish flour have influence on the nucleic acid metabolism.