1. 1. Polyamines and glutathione were determined in tissues of lactating and normal female rats and in an ascites form of rat Rd/3 sarcoma. 2. 2. During lactation, rat liver glutathione decreased and spermidine/spermine increased. Similar but more pronounced changes had previously been found in livers of pregnant rat or rats bearing solid Rd/3 sarcoma. 3. 3. Lactating mammary tissue has a very high spermidine content and sperdimine/spermine ratio, similar to values found in a mouse ascites tumour by other workers. However, the ascites form of Rd/3 sarcoma had the same lower polyamine and glutathione levels as the solid Rd/3 tumour. © 1968.