New hydroxycinnamic acid esters have been isolated from the cotyledons of Amaranthus cruentus.(E)-Caffeoylisocitric acid was identified as the major constituent and pcoumaroyl- and feruloylisocitric acids as minor ones on the basis of 'H and 13C NMR spectroscopy. FAB mass spectrometry, high-performance liquid and thin-layer chromatography. The structure of caffeoylisocitric acid was confirmed by chromatographic comparison with synthetic material. The accumulation of the new hydroxycinnamoylisocitric acids and their enzymatic synthesis via the hydroxycinnamoyl-CoA thioesters as acyl donors are described. The caffeoyl-CoA-dependent acyltransferase showed a rapid transient increase in activity reaching ca 5 pkat per cotyledon pair (i.e. ca 390 pkat per mg protein) at day six of seedling development.