Homocysteine, an independent risk factor for cardiovascular disease, is methylated in the liver via the zinc metalloenzyme betaine-homocysteine methyltransferase (BHMT). Established assays for BHMT include a radiochemical assay, a colorometric assay, an HPLC assay and an in vivo microbiological assay. These techniques are either unsuitable for substrate specificity studies, or are unable to give kinetic measurements. BHMT was purified from liver and measured directly and kinetically by a novel ¹H-NMR spectroscopic assay. The disappearance of substrates and the formation of products are monitored simultaneously. Using 2 mM glycine betaine and homocysteine as substrates in 20 mM phosphate buffer (pH = 7.5) and measuring the production of N,N-dimethylglycine the CV is 6.3% (n = 6) and the detection limit is 6 nkatal. An endpoint assay for BHMT activity was also developed and had CV = 5.3%, n = 6, with a detection limit of 2 nkatal. The NMR spectroscopic assay was used to determine the substrate specificity with a library of alternative substrates. Analysis of betaine analogues with different chain length, α-substitution, substitution of the nitrogen and carboxyl moieties demonstrated that BHMT is inactive if there is any steric crowding of the nitrogen or α-carbon positions. BHMT is capable of using group VI heteroatom betaines as methyl donors, with much faster rates than glycine betaine. For glycine betaine the Km was 0.19 ± 0.03 mM with a Vmax of 17 ± 0.7 nMol min-1 mg-1. The same assay was used to detect and partially characterise a BHMT activity from hagfish liver that is similar to that of the mammalian enzyme. NMR spectroscopy was adapted for measurements of glycine betaine in urine, along with other medically significant methylamines. These were shown to be valid for clinical use and in animal studies. A novel metabolite of the sulfonium analogue of glycine betaine (methylsulfinylmethanoate) was identified in rats.