A high-pressure liquid chromatographic procedure for the simultaneous analysis of N-nitroso and N,N;dinitroso derivatives of piperazlne Is described. Its application in assaying these toxic nitrosamines formed under in vitro conditions in human saliva and in vivo conditions In rat stomach Is reported. Piperazine is a highly effective and widely used anthelmintic agent in the management of round worm (Ascaris lumbricoides) and pin worm (Oxyuris vermicularis) infections (1). Recently, piperazine was found to react with sodium nitrite under in vitro and in vivo conditions to yield N-nitroso (MNP) 1 and N,N'-dinitroso (DNP) derivatives (2-6). Both MNP and DNP were carcinogenic to mice and rats (7-11). Concurrent oral administration of piperazine and sodium nitrite to mice produced lung adenomas attributable to in vivo nitrosation of the drug (7). The recommended maximum oral dose of piperazine (as its adipate, calcium editate, citrate or tartrate salt) is 3.5g/day for 2 days for ascariasis and 2.Sg/day for 8 days for oxyuriasis. Because of this high intake of a readily nitrosatable drug, some investigators have expressed concern regarding its nitrosative toxification in the stomach, especially in the absence of proper attention to dietary intake of nitrite (4,6,7). Recently, a spectrophotometric (12) and several thin-layer (4,6) and gas chromatographic (2,5,13) methods were reported for the analysis of N-nitroso derivatives of piperazine. The present paper describes a convenient and rapid high-pressure liquid chromatographic (HPLC) procedure for the simultaneous analysis of MNP and DNP formed under in vitro and in vivo conditions.