The total free amino acid content per gram of liver wet weight does not differ significantly between adult and senescent female rats, being 28.8 ± 3.3 µmol in adult (13–17 months) and 26.2 ± 2.2 µmol in senescent (23–29 months) animals. This is the first study in which these two age-groups of rats have been so compared. These results combined with a survey of the literature show that amino acid levels in rat liver do not change much, if at all, throughout the lifespan of the