Putrescine N-methyltransferase (PMT, EC 220.127.116.11) catalyses the S-adenosyl- L-methionine (SAM or AdoMet)-dependent methylation of putrescine to N-methylputrescine within the biosynthetic pathways of calystegines, nicotine, and tropane alkaloids in medicinal plants and produces S-adenosyl- L-homocysteine (SAH or AdoHcy). Determination of PMT activity was time-consuming and hardly reproducible in the past because it required tedious separation steps after chemical derivatisation or radioactive labelling of N-methylputrescine. A convenient and accurate enzyme-coupled colorimetric assay is based on the conversion of SAH to homocysteine by 5'-methylthioadenosine/ S-adenosylhomocysteine nucleosidase (MTAN/SAHN, EC 18.104.22.168) and S-ribosylhomocysteine lyase (LuxS, EC 22.214.171.124). Homocysteine is quantified by 5,5'-dithiobis-2-nitrobenzoic acid. Putrescine was shown not to interfere with MTAN or LuxS. The colorimetric assay was validated by HPLC analysis. K(m) values determined by the assay, 108 microM for putrescine and 42 microM for SAM, are lower than the previously reported values, due to alleviation of PMT inhibition by SAH. DTNB:5,5'-dithiobis-2-nitrobenzoic acid LuxS: S-ribosylhomocysteine lyase MTAN:5'-methylthioadenosine nucleosidase PMT:putrescine N-methyltransferase SAH: S-adenosyl- L-homocysteine SAM: S-adenosyl- L-methionine TNB:2-nitro-5-thiobenzoic acid.