SummaryAnalyses of brain and carcass of normal and lathyritic chick embryos for free hydroxyproline, proline, glycine, and total ninhydrin reactive material are reported. At 10 days development, 6 days after BAPN administration, no obvious alterations in these parameters were found although embryos were grossly malformed. At 17 days of development, 13 days after treatment, a reversal in ratios of proline to hydroxyproline was found; proline concentration fell, and that of hydroxyproline rose in both brain and carcass of lathyritic embryos as compared to the normals. A rise in free glycine concentration resulted in unchanged ratio of glycine to hydroxyproline.