A barley (Hordeum vulgare L.) mutant deficient in serine:glyoxylate aminotransferase (SGAT) was crossed with wild-type plants to generate heterozygous mutants. Plants of the F2 generation with reduced SGAT activities (45-60% of wild-type activities) contained proportionally less SGAT protein. Reduced SGAT activities resulted in the accumulation of serine and, to a smaller extent, of glycine, indicating that the flux through the photorespiratory pathway was restricted. Rates of photosynthesis were, however, not affected by the reduction in SGAT activity.