The effects of sulphate nutrition on watercress were investigated with respect to visual deficiency symptoms, growth response and flavour content as determined by sensory and gas-chromatographic methods. The plants were grown in a glasshouse, both in solution culture using plants propagated vegetatively and in sand culture using plants raised from seed, at six concentrations of sulphate in the nutrient medium. The two methods were complementary and led to substantially similar conclusions. There were steep increases in weight per plant, sulphur content and flavour strength with initial increases of sulphate concentration in the nutrient medium, followed by levelling off- at the highest sulphate concentrations. Sulphur requirements for maximum growth were satisfied at lower sulphate concentrations (0.20-0.25 meq/1) than those for maximum flavour development (0.75 meq/1). There is evidence that watercress flavour consists of at least two types of components: (a) components which impart a bland, generalized vegetable taste and odour and (b) a specific component or components which are perceived as a burning sensation in the mouth accompanied by a characteristic odour. Production of the former is independent of sulphur nutrition. Enzymatic production of ß-phenylethyl isothiocyanate from the corresponding glucosinolate is believed to be responsible for the latter. There were highly significant correlation coefficients between the isothiocyanate content of fresh tissues and their total and organic sulphur contents. The possibility of controlling watercress flavour in commercial production, with a view to meeting consumer preferences, is briefly discussed.