Abstract: N-Formylsarcolysine (NF) labelled with C¹⁴ in the formyl position (NF-C¹⁴) was used in this investigation to study the absorption and excretion of radioactivity in four patients suffering from seminoma. Seventy-two hours after an oral dose of 15mg of NF-C¹⁴ (about 30 μC, one patient was administered 100mg of unlabelled NF in addition to the labelled compound), 36.6, 33.2, and 34.1% of the administered radioactivity were excreted in the urine in three patients. However, most (89-97%) of the urinary radioactivity was excreted in the first 5-hour urine. During the 72-hour period, 33.5, 36.2, and 43.3% of the dose were recovered in the faeces of these patients, and of these, 86-99% was accounted for in the first 48-hour specimen. The fourth patient who had constipation excreted 28.8% of the administered radioactivity in the urine in 72 hours, most (92%) of which was re- covered in the first 5-hour urine sample. About 39.2% was recovered from the faeces in 6 days, mainly (98%) in the first 4 days (including the first and the second faeces specimen voided since the drug administration). Only trace of radioactivity was detected in the blood, saliva, and expired carbon dioxide in the first 24-hour period after NF-C¹⁴ administration. These results indicate that, in patients given NF-C¹⁴ orally, the radioactivity was ab- sorbed rapidly but incompletely. However, the possibility that the absorbed radioactivity reentered the intestinal tract and was excreted in the faeces should not be excluded. It appears that the absorption and excretion of radioactivity after per oral NF-C¹⁴ was not affected by the simultaneous administration of a large dose of the unlabelled drug.