When 400 mg/kg of 14C-methyl-labeled N-methylformamide (NMF) was injected ip into mice, the curve for plasma concentration of radioactivity versus time was superimposable on the curve obtained by measuring unmetabolized NMF with gas-liquid chromatography during the first 24 hrs. Radioactivity in plasma was measurable for 8 days after NMF administration, but NMF was not measurable by gas chromatography beyond 24 hrs after administration. Radioactivity was eliminated from the plasma after 60 hrs, with an apparent half-life of 71.1 hrs. Of the radioactivity injected with NMF, 73.6% was recovered in the urine in 24 hrs; 26.4% of this was unchanged NMF. Three percent of the administered radioactivity was exhaled as 14CO2 in 7 hrs at a constant rate of 0.007% per min. One urinary metablite was a stable precursor of formaldehyde, which decomposed to formaldehyde only after alkaline hydrolysis and may well be N-(hydroxymethyl)-formamide. The areas under the plasma concentration versus time curve were estimated after ip, iv, and oral administration of NMF. The bioavailability of NMF was 1.01 after oral administration and 1.10 after ip administration.