Recent reports have suggested the involvement of arachidonic acid (AA) and its metabolites in the control of body pattern, head and tentacle regeneration and bud formation in Hydra spp. Here we describe for the first time the biosynthesis of hydroxyeicosatetraenoic acids (HETEs) in vitro by hydroid cytosolic extracts. Incubation of both unlabelled and tritiated AA with homogenates of Hydra vulgaris led to the conversion of up to 11% of the exogenous fatty acid into mainly two metabolites. These were characterized as 11-hydroperoxyeicosatetraenoic acid (11-HPETE) and 11-HETE by means of a combination of chromatographic, chemical, 1H-n.m.r. and electron-impact m.s. techniques. Trace amounts of 9-HETE and 12-HETE were also found. Analysis of 11-HETE by chiral-phase h.p.l.c. revealed that this metabolite was composed mainly of the R enantiomer. The production of 11-HPETE and 11-HETE was found to be: (1) associated with the cytosolic fraction of Hydra homogenates; (2) dependent on AA concentration, incubation time and protein amount in the homogenates; (3) unaffected by co-incubation with the 5- and 12-lipoxygenase inhibitors, 5,8,11-eicosatriynoic acid and nordihydroguaiaretic acid, the cyclo-oxygenase inhibitor, indomethacin, or the cytochrome P-450 inhibitors, proadifen and methoxalen. These results strongly suggest the presence of a very active (R)-11-lipoxygenase in H. vulgaris. The activity of both R and S enantiomers of synthetic 9-, 11- and 12-HETE and of 'endogenous' 11-HETE was studied on tentacle regeneration and bud formation in decapitated Hydra. Although almost all compounds tested inhibited budding, only endogenous 11-HETE and synthetic (R)-11-HETE significantly enhanced the average number of tentacles, thus suggesting that this eicosanoid might be one of the cellular regulators of regeneration in H. vulgaris.