Rats were fed four levels of threonine (Thr, 0.4, 0.6, 0.8, and 5.8 g/100 g diet). After two weeks, Thr, serine (Ser), and glycine (Gly) levels were measured in plasma, liver, muscle, and central nervous system. The diet containing 5.8 g/100 g of Thr elevated Thr and Gly concentrations in plasma and nervous tissue in comparison with a standard diet. In muscle and liver, Thr concentrations were also raised but Gly levels did not change. The hepatic Thr dehydratase activity was enhanced. Diets containing moderate Thr quantities (0.6 and 0.8 g/100 g) induced slight elevations of Thr levels in all tissues. Gly concentrations were not modified. The activity of hepatic Thr dehydratase was diminished. Our results show that a high dietary content of Thr (15 times the normal levels) elevates Gly levels in various tissues, including the brain. On the contrary, diets containing 2 to 4 times the normal levels of Thr induce a weak hyperthreoninemia insufficient to modify brain Gly.