IT was shown [Brown, 1938] that uric acid, or its oxidation product allantoin, is being constantly produced, as a nitrogenous end-product by Lucilia sericata throughout its life cycle. It remains to elucidate the role of NH3, known to be produced in large quantities by larvae of this species, and to investigate the possible occurrence of any other catabolite. This paper reports studies on the subjects of NH3 and of its relationship with uric acid and allantoin, of urea, creatine and creatinine, and other nitrogenous substances.