The amdS gene codes for an acetamidase enzyme that hydrolyses acetamide to acetate and ammonium thus providing A. nidulans with a source of carbon and nitrogen. The exceptionally favourable genetics of this system combined with molecular analysis have enabled many regulatory circuits affecting amdS to be identified genetically. Characterization of the regulatory genes and the definition of the cis-acting sites involved have been done using both in vivo and in vitro mutagenesis. Recent results on the analysis of the system are presented.