Recent reports have shown the occurrence of regiospecific and enantioselective lipoxygenase-mediated metabolism of arachidonic acid (AA) in cytosolic extracts of marine and freshwater hydroids. Here we report that cytosolic extracts of Hydra magnipapillata are unique among hydrozoans for their capability of converting AA into two major metabolites which showed chromatographic, mass spectrometric and nuclear magnetic resonance properties identical to those of 11-R- and 12-S-hydroxyeicosatetraenoic acid (11-R-HETE and 12-S-HETE). The production of neither compound was affected by co-incubation of H. magnipapillata extracts with the cytochrome P-450 inhibitor proadifen. The 5- and 12-lipoxygenase inhibitor nordihydroguaiaretic acid (NDGA), while inhibiting 12-S-HETE formation at high concentrations, did not influence 11-R-HETE production, thus suggesting the co-localisation, unprecedented in hydroids, of two separate enantioselective lipoxygenase-like activities. The possible role of the two metabolites in the control of hydroid body pattern was investigated. At low micromolar concentrations, both enantiomers of 11-HETE inhibited diacylglycerol-induced ectopic head formation (EHF), while 12-S-HETE, and its likely precursor 12-S-hydroperoxy-eicosatetraenoic acid (12-S-HPETE), enhanced bud formation, thus providing the first example of endogenous metabolites controlling, respectively, hydroid 'head activation potential' and asexual reproduction.