1. The following compounds were glucuronidated in the presence of UDP-glucuronic acid and a microsomal preparation made from guinea-pig liver: ¹⁴C-labelled 3-O-methyladrenaline, 3-O-methylnoradrenaline, 3-methoxytyramine and 4-hydroxy-3-methoxyphenethanol, as well as unlabelled harmalol and harmol. 2. [¹⁴C]Homovanillic (4-hydroxy-3-methoxyphenylacetic) acid was not a substrate for the microsomal glucuronyltransferase. 3. The K_m values for harmalol and harmol were 0·69×10⁻⁴m and 0·50×10⁻⁴m respectively. 4. The K_m values for UDP-glucuronic acid, in the presence of ¹⁴C-labelled 3-O-methylnoradrenaline, harmalol and harmol as aglycones, were 0·57×10⁻⁴m, 0·44×10⁻⁴m and 2·20×10⁻⁴m respectively. 5. Mg²⁺ added at 2·5–10mm activated glucuronyltransferase, with harmalol as substrate. Concentrations above 10mm inhibited the enzymic activity. 6. The overall, or net, transglucuronidating activity of microsomal preparations of the liver, with harmalol as substrate, was greatest for guinea pig, and very much lower for rabbit, mouse and rat.