The aim of this study was to ascertain the possible toxicological effects of chemicals released into mineral water packaged in polyethylene terephthalate (PET) bottles. Two commercial mineral waters, bottled both in PET and glass and stored under different conditions, were examined using theAllium cepatest. The influence of the water samples on macroscopic (root length, colour and form) and microscopic (root tip mitotic index, chromosome aberrations) parameters was examined. The water samples were analysed after: (A) controlled-conditio n storage (no direct light exposure and 18 ± 2°C), (B) storage at 40|°C for 10 days, in the dark (migration test in accordance with 82/711/EEC), and (C) exposure to sunlight and varying temperatures (18 ± 38°C, mean temperature 25 ± 38°C). The two water samples bottled in PET induced cytogenetic aberrations regardless of the storage conditions. These signs of toxicity were evident even only 8 weeks after bottling, which is well within the recommended expiry date. Storage conditions were very important, as is suggested by the finding that chromosomal aberrations were particularly apparent after exposure to direct sunlight. However, as plant systems are not considered as primary screening tools by current international guidelines for mammalian systems, extrapolation of the results from this test system to other systems and, eventually, to human beings should be based on results from a battery of assays covering various metabolic pathways.