synopsis. Pigmentary mutants in amphibians provide important vehicles for studying basic problems in development. Some of these mutants exert influences on the tissue environ? ment in which the chromatophores differentiate and others involve the expression of pigment in specific types of pigment cells. Melanophores are the best known of all chromatophores, and albinism has been much studied. Genes controlling its expression may operate at different levels. Some are involved in the production of tyrosinase, while others affect the melanosomal matrix. In contrast, melanoid mutants are characterized by an overproduction of eumelanin, usually through the differentiation of an excessive number of melanophores. Melanoid mutants of the axolotl also exhibit a great diminution in xanthophore and iridophore number, and the same seems to be true of some melanoid-like mutants of leopard frogs. The pteridine pigments of xanthophores and purines of iridophores are often products of xanthine dehydrogenase (XDH) activity, and it is suspected that the expression of the melanoid phenotype may result from a genetic defect involving this enzyme. This is supported by experiments involving the administration of an XDH inhibitor, allopurinol, to normal larvae. This inhibitor results in the production of partial phenocopies of the melanoid condition. Blue mutants involving either partial or complete diminution of xanthophore and iridophore pigments may also be based upon deficient XDH activity.