In vitro and animal studies suggest that compounds found in garlic may play a role in mediating the bioactivation of some carcinogens. N-nitrosoproline (NPRO) has been used in feeding studies as a biomarker for the formation of N-alkylnitrosamines. A controlled-feeding study was performed at Penn State University to test the hypothesis that garlic consumption inhibits nitrosamine formation. In this study, subjects (3 to 5 per group) consumed either 0 (control) 1, 3 or 5 grams of fresh garlic, 3 grams Kyolic (a commercial garlic supplement) or 500 mg vitamin C for 1 day following a 3 day run-in period in which 300 mg nitrate and 500 mg proline was consumed daily with a standard meal. Urine was collected during the last 24 hours and analyzed for NPRO. Aliquots were diluted with mobile phase, fractionated by isocratic reversed phase HPLC, and the fraction containing NPRO was dried and derivatized with pentafluorobenzyl bromide (PFB). The NPRO-PFB derivative was isolated using a second, gradient reversed-phase HPLC step and quantified by gas chromatography/ negative chemical ionization mass spectrometry. 13C5NPRO was used as the internal standard. Preliminary analysis of the samples indicates that NPRO excretion was reduced to nearly undetectable levels by treatments with 5 grams garlic, 3 grams Kyolic, and 500 mg vitamin C compared to controls. Analysis of the remaining samples is underway. In conclusion, the consumption of a single dose of prepared garlic, either fresh or as a supplement, reduced the excretion of NPRO under controlled-feeding conditions as much as a vitamin C supplement.