1. We have measured the incorporation of an intraperitoneal injection of [3H]glutamate into the protein of the gut, liver and kidney of lean and obese siblings of the genetically obese mouse.
2. Recycling of the 3H was minimized by using glutamate labelled at the C-2 position. Loss of label from the amino acid pool by transamination and deamination was rapid, with a half-life of 4 h.
3. In tissue protein the amino acid showing the highest 3H radioactivity was glutamate.
4. The half-lives for protein synthesis and catabolism were calculated from the decay curves of both specific and total radioactivity of [3H]glutamate in tissue protein. No significant differences were found between kidney, liver and gut in lean and obese mice.