In the reductive pyrimidine catabolic pathway uracil and thymine are converted to beta-alanine and beta-aminoisobutyrate. The amidohydrolases of this pathway are responsible for both the ring opening of dihydrouracil and dihydrothymine (dihydropyrimidine amidohydrolase) and the hydrolysis of N-carbamyl-beta-alanine and N-carbamyl-beta-aminoisobutyrate (beta-alanine synthase). The review summarizes what is known about the properties, kinetic parameters, three-dimensional structures and reaction mechanisms of these proteins. The two amidohydrolases of the reductive pyrimidine catabolic pathway have unrelated folds, with dihydropyrimidine amidohydrolase belonging to the amidohydrolase superfamily while the beta-alanine synthase from higher eukaryotes belongs to the nitrilase superfamily. beta-Alanine synthase from Saccharomyces kluyveri is an exception to the rule and belongs to the Acyl/M20 family.