In addition to reviewing the genetic regulation of L-glycerol 3-phosphate dehydrogenase during development in the mouse, new evidence is presented that the electrophoretic properties of L-glycerol 3-phosphate dehydrogenase in M u s castaneus are determined by an allele ( d j at the Gdc-l locus. Accordingly there are three alleles at the Gdc-1 locus; the b allele in C57BLl6J mice differs from the d allele in electrophoretic properties and the c allele in BALBlcJ mice differs from the d allele with respect to both heat denaturation and electrophoretic properties. Identical segregation patterns of the L-glycerol 3-phosphate dehydrogenase phenotypes in liver, kidney, and skeletal muscle from offspring of an F2 generation produced from parents with the cld genotype suggest that the Gdc-1 locus is the major structural locus for L-glycerol 3-phosphate dehydrogenase in these tissues. Heart muscle was pooled from mice of the F2 generation with either clc or dld genotypes at the Gdc-1 locus as determined by analysis of liver L-glycerol 3-phosphate dehydrogenase. The chromatographic properties of L-glycerol 3-phosphate dehydrogenase from the heart muscle was determined on DEAE-cellulose ion exchange columns. The elusion profile of the L-glycerol 3-phosphate dehydrogenase indicates that the Gdc-1 locus is also the major structural locus in heart muscle. ~~ ~ ~