Watercress (Nasturtium officinale R.Br.) is the richest source of glucosinolate nasturtiin, which on hydrolysis produces phenethyl isothiocyante (PEITC). Interest in growing watercress is stimulated since demonstration of the role of PEITC in protection against cancers associated with tobacco specific carcinogens. Twenty-one days old watercress seedlings were transplanted into growth chambers (16-h days/8-h nights of 25/22 degrees C and photosynthetic photon flux (PPF) of approximately 265 micromol m(-2) s(-)(1)). The study was replicated three times. Leaves were analyzed for PEITC and ascorbic acid concentrations at transplant, and harvested at 10-days intervals until 60 days after transplant. The PEITC and ascorbic acid concentrations were the highest in leaves harvested at 40 days and the lowest at transplant. Leaves harvested at 40 days produced about 150% higher PEITC concentrations compared to the leaves at transplant. Both PEITC and ascorbic acid concentrations of leaves increased linearly with age until 40 days after transplant after which there was no significant increase. Seedlings at transplant had the lowest dry mass and leaf area, while plants harvested at 60 days had the highest dry mass and leaf area.