Abstract 10-Methoxydeserpidine caused a slow sustained fall in the arterial blood pressure of the anaesthetised cat when doses of 2 to 4 mg./kg. were given, an effect seen only when the initial blood pressure was high. Unlike reserpine and deserpidine, 10-methoxydeserpidine did not cause ptosis or loose stools in mice or rats when used at doses of from 10 to 80 mg./kg. and although pentobarbitone sleeping time was not increased, at higher doses of 40 to 80 mg./kg. there was reduction of motor activity, respiratory depression and some animals died. Unlike deserpidine the 10-methoxy derivative had no effect on the pressor responses to compression of the abdominal aorta or stimulation of the splanchnic nerve, but depressed the response to occlusion of the common carotid arteries. The LD50 of 10-methoxydeserpidine was found to be 82 ± 2–6 mg./kg. in mice. Methyl reserpate and reserpic acid showed reserpine-like activity in intact animals and isolated tissues only at very high doses.