Catalytic addition of two molecules of hydrogen to 7-deuterio-6-trideuteriomethylpterin yields a 0.8 : 1 mixture of cis- and trans-7-deuterio-6-trideuteriomethyl-5,6,7,3-tetrahydropterin. Similar reduction of 2,4-diamino-7-deuterio-6-(partial)trideuteriomethylpteridine gives a 1 : 1 mixture of cis- and trans-2,4-diamino-7-deuterio-6-(partial)trideuteriomethyl-5,6,7,8-tetrahydropteridine. Catalytic reduction of 6,7-dimethyl-and 6,7-bis(trideuterio-methyl)pterin, on the other hand, is stereospecific and forms only the cis-5,6,7,8-tetrahydro-derivatives. Reduction of 6,7-dimethylpterin with sodium in ethanol provides a 1 : 1 mixture of cis- and trans-6,7-dimethyl-5,6,7,8-tetra-hydropterin. The stereochemistry of these products was deduced from 1H n.m.r. spectroscopy.