Three allelic mutants of Arabidopsis thaliana which lack mitochondrial serine transhydroxymethylase activity due to a recessive nuclear mutation have been characterized. The mutants were shown to be deficient both in glycine decarboxylation and in the conversion of glycine to serine. Glycine accumulated as an end product of photosynthesis in the mutants, largely at the expense of serine, starch, and sucrose formation. The mutants photorespired CO(2) at low rates in the light, but this evolution of photorespiratory CO(2) was abolished by provision of exogenous NH(3). Exogenous NH(3) was required by the mutants for continued synthesis of glycine under photorespiratory conditions. These and related results with wild-type Arabidopsis suggested that glycine decarboxylation is the sole site of photorespiratory CO(2) release in wild-type plants but that depletion of the amino donors required for glyoxylate amination may lead to CO(2) release from direct decarboxylation of glyoxylate. Photosynthetic CO(2) fixation was inhibited in the mutants under atmospheric conditions which promote photorespiration but could be partially restored by exogenous NH(3). The magnitude of the NH(3) stimulation of photosynthesis indicated that the increase was due to the suppression of glyoxylate decarboxylation. The normal growth of the mutants under nonphotorespiratory atmospheric conditions indicates that mitochondrial serine transhydroxymethylase is not required in C(3) plants for any function unrelated to photorespiration.