N-Nitrosoproline (NPRO) formed intragastrically after ingestion of 8 mg/kg body weight sodium nitrate and the same amount of proline was determined in the sweat of six volunteers during intensive sweating in a Finnish sauna (60-80 degrees C). The amounts of sweat and NPRO and the concentrations of this compound varied between individual volunteers and in the same subjects in consecutive experiments. Females excreted higher concentrations of NPRO in smaller amounts of sweat. During the second hour after the start of intragastric nitrosation of proline, the concentrations and amounts of NPRO in sweat decreased rapidly.