Terence Mansfield is noted for his contributions to the physiology of stomata and especially the chemical aspects of stomatal mechanism, often made in collaboration with his students in the active and successful research school in plant physiology that he established in 1967. His important findings include the discovery of a new ‘low energy’ light effect on stomata, with maximum sensitivity at 700 nanometres, 0.1 Jm-2s-1 being effective in shifting the phase of an endogenous rhythm in darkness. With a research student he also discovered that abscisic acid (ABA) acts as a stress hormone, suppressing stomatal opening with concomitant prevention of K+ uptake and starch hydrolysis in the guard cells. A simple bioassay was developed, sensitive from 10-11 to 10-4 M ABA, which was also suitable for other substances with ‘antitranspirant’ properties; one of these (all-trans-farnesol) was discovered in drought-stressed Sorghum leaves. He has also made other important advances in techniques for the study of stomata, both in intact leaves and in isolated epidermis.