Abstract.-This paper describes the synthesis and biological activity of [5-L-asparagine-a4,34,3-d3]-deamino-oxytocin. The model peptide S-benzyl-3-mercaptopropionyl-L-asparaginyl-a4,3,-d3-glycinamide was also prepared to observe the effect of the conditions of peptide synthesis on the deuterium content. It was found that normal synthetic procedures could be used without a significant loss in the deuterium content.The [5-L-asparagine-a,,S,-d3 1-deamino-oxytocin had, within experimental error, the same avian vasodepressor and oxytocic activities as deamino-oxytocin itself.In a continuation of a program to evaluate the effect on the biological activity of deamino-oxytocin (Fig. 1) (1) Synthesis of N-carbobenzoxy-L-asparagine-a,,f-d3 (II) and p-nitrophenyl N-carbobenzoxy-L-asparaginate-a,fid-d3 (III): The procedure reported for the protio analog was followed.4 L-Asparagine-a,#,#-d3,5 2.0 gm (0.015 mole), 3.0 gm of potassium bicarbonate and 2.68 gm of benzyl chloroformate in 40 ml of water gave 3.2 gm (89%) of the acid II: mp 160(161' (cor). The acid II was dissolved in 32 ml of ethanol, filtered through glass, cooled immediately,4 and dried to give 2.48 gm (69%) of the acid II:3 mp 165-166°(cor);[a]D23-6.2' (c 1, 1 N NaHCO3). Mass spectrum: d2, 12.3%; d3, 87.7% (96% D).A modification of the procedure reported for the protio analog was followed.4 A solution of 2.30 gm (0.008 mole) of the acid II, 1.45 gm of p-nitrophenol, and 1.95 gm of dicyclohexylcarbodiimide (DCC) in 18 ml of dimethylformamide (DMF), dried and freshly distilled,6 was stirred for 1 hr at 00 and at room temperature for 15 min. Glacial acetic acid (0.3 ml) was added, the solution was stirred for 15 min, cooled to -20°, and filtered. The product III was precipitated by the addition of 70 ml of water, and was isolated by filtering the cooled mixture. After being dried in vacuo for 24 hr, there was 263