Human nitrosating capacity has been monitored using proline; however, N-nitrosothiazolidine 4-carboxylic acid (NTCA; N-nitrosothioproline), one of the predominant N-nitroso compounds in human urine, is also nonmutagenic and, presumably, noncarcinogenic. Thioproline is nitrosated about 1000 times faster than proline in vitro, and NTCA is excreted into the urine without being metabolized. We have therefore proposed thioproline as an effective nitrite-trapping agent in the human body. Recently, we found thioproline in various cooked foods, including cod and dried shiitake mushrooms. In the study reported here, we evaluate the nitrite trapping capacity of thioproline in a male nonsmoking volunteer ingesting NO3- and eating a controlled diet. The highest level of NTCA excreted, 5.89 mumol, was measured after the subject ingested 6 mmol NO3- followed by 0.45 mmol (60 mg) thioproline. We estimated the effective amount of nitrite, defined as the actual amount of nitrite participating in nitrosation in the stomach, to be 0.3% of the NO3- ingested. Thus, the effective amount of NO2- for 6 mmol NO3- ingested was calculated to be 18 mumol, and 33% of this nitrite was trapped by ingestion of 0.45 mmol thioproline. We conclude that thioproline is a most sensitive probe for evaluating human nitrosating capacity and an effective nitrite-trapping agent.