A "new" carbon radioisotope, 11C, for use in nutritional studies is presented. It has a 20 min half-life, and decays by positron emission giving annihilation photons of 511 keV energy (Wolf & Redvanly, 1977). Thus repeated studies can be made with short time intervals and the distribution of radioactivity in the experimental animal can be detected externally. 2. 11C was produced with a tandem Van de Graaff accelerator and L-[me-11C]methionine was synthesized and used in model experiments in the pig. The tracer was administered intravenously through a catheter in the jugular vein of pigs weighing between 40 and 100 kg. In a series of experiments, one pig received a low-methionine diet supplemented with DL-methionine to give three different levels of methionine intake. 3. The radioactivity distribution between liver and muscle was measured as a function of time by external detector for 2-3 h after administration. Blood and exhaled CO2 were sampled and measured for radioactivity. 4. The results indicate that 11C is a useful radionuclide in nutritional studies in intact large domestic animals.U
