Hypersensitive browning and phytoalexin accumulation were induced in potato tuber (Solanum tuberosum L.) by the elicitor arachidonic acid. Phenylthiourea, an inhibitor of polyphenoloxidases, completely inhibited browning of the tissue induced by the elicitor in old (8 months) but not in young (1-2 months) tubers. Phytoalexin accumulation was not affected by the action of phenylthiourea both in young and old tubers, showing that this aspect of hypersensitive response is not necessarily related to the browning of the tissue. Hydroxyl radical (HO.) induced browning of the tissue but no phytoalexin accumulation. Gas chromatographic-mass spectrometric analyses showed that the rate of metabolization of arachidonic acid was independent of the addition of phenylthiourea and of the age of tubers.