Aliphatic amines, such as mono-, di- and trimethylamine (MMA, DMA, TMA) and trimethylamine N-oxide (TMAO), are produced by bacterial degradation of trimethylalkylammonium compounds, such as choline, acetylcholine, carnitine and γ-betaines. This degradation can be environmental, intestinal or faecal1.2. This paper describes an analytical investigation by gas chromatography (GC) with flame ionization detection (FID) or thermionic specific detection (TSD) and by high-performance liquid chromatography (HPLC) with conductimetric, refractive index (RI) and electrochemical detection (ED) for the simultaneous determination of the above amines, mainly applied to biological samples such as urine, and an assay procedure adopted consisting in chemical reduction of TMAO to TMA, followed by evaluation of the latter by GC in order to determine the TMAO. © 1990.