Formation of N-nitrosopyrrolidine (N-Pyr) from proline and N-nitrosoproline (N-Pro) was investigated by cooking pork slices to which these precursors had been added. When the cooking system was heated in an oil bath at 200 C for 12 min, 0.33 and 2.18% yields of the N-nitrosamine were obtained from proline and N-Pro, respectively. The N-Pro contents of pork slices to which two levels of nitrite were added were determined after 1 and 8 days of storage at 2 C. Results indicate that the formation of N-Pro occurs at such a rate that 1 day after addition of nitrite there is theoretically sufficient N-Pro formed to account for the quantities of N-Pyr isolated from cooked bacon. However, the rate of decarboxylation of the initial N-Pro in raw bacon is not great enough to account for the N-Pyr isolated from cooked bacon.