1. 1. The hepatopancrease of twelve gastropods was checked for the ornithine cycle enzymes and urease. With the colorimetric assay only the prosobranchs, Marisa cornaurietis and Sinotia ingallisiana, were shown to biosynthesize citrulline. These data, along with reports of other workers using [ 14C]-bicarbonate incorporation into citrulline and urea, suggest that both prosobranchs and pulmonates probably are able to slowly synthesize citrulline. 2. 2. Although all of the snails possessed arginase and ornithine transcarbamylase, activity was consistently higher in pulmonates than in prosobranches. The overall reactions also was lower or absent in prosobranchs. Apparently arginine and urea biosynthesis occurs to some extent according to the ornithine cycle or a similar cycle in many snails. Urea biosynthesis seems of special importance during estivation of some pulmonates. 3. 3. Urease was found in Achatina fulica, Bulimulus dealbatus, Helix aspersa and Euglandina singleyana tissue, but its biological importance has not been established.