Since asparagine has been found to inhibit growth of some tumors and to inhibit or delay mitotic activity in other cells, we have studied the effect of asparaginase and of deprivation of some essential amino acids (Arg, Asn, Leu, Ile, Trp) on nucleic acid and protein synthesis in an asparagine-requiring strain of BHK/21 cells. We find that: (1) there is no essential difference in the pattern of synthesis following deprivation of any of the amino acids we tested; (2) that the effect of asparaginase is similar to that of amino acid deprivation; (3) that RNA synthesis is inhibited more rapidly than DNA or protein synthesis; (4) that after 10 hr of amino acid starvation, DNA synthesis is almost totally (reversibly) inhibited while RAN synthesis continues at about 30-50% and protein at about 100% of the initial value.