SummaryGlycine transport in both normal and nonketotic hyperglycinemia fibroblasts was shown to occur by a sodium-dependent system. No significant difference could be detected in either the Km's (1.4 to 2.0 mM) or the Vmax's (6.2 to 16 nmole per mg protein per min) of the three control and three patient cell lines. Valine was a weak competitive inhibitor of glycine uptake. Kis from both groups fell into the 5.6 to 5.8 mM range. Plasma levels of valine of one patient reached a maximum of 0.6 mM following a valine load. Glycine cleavage activity could not be detected in either control or nonketotic hyperglycinemia fibroblast lines. Serine utilization was the same in both nonketotic hyperglycinemia and control lines.