Endogenous formation of N-nitrosoproline (NPRO) was demonstrated by monitoring its excretion in the urine of a male volunteer who had ingested vegetable juice, as a source of nitrate, and proline. The resulting NPRO was analyzed after derivatization by combined gas-liquid chromatography thermal energy analysis. The amount of total NPRO excreted in the urine was found to be proportional to the proline dose and increased exponentially with the nitrate dose ingested. Neither nitrate nor proline, when taken alone, led to a detectable increase in NPRO in urine. The amounts of NPRO formed (as estimated from the amounts excreted within 24 hr) after dosing 325 mg nitrate (NO3-) followed by 500 mg proline, ranged from 16.6 to 30.0 (mean, 23.3) micrograms per person. The simultaneous intake of ascorbic acid or alpha-tocopherol inhibited nitrosation of proline in vivo. Monitoring of NPRO or other N-nitroso compounds excreted in the urine thus appears to be a suitable procedure for estimating daily human exposure to endogenously formed N-nitroso compounds.