Lithium ions affect pattern formation in the freshwater polyp Hydra vulgaris in a complex manner. Although a long-term treatment with 1 mM LiCl completely suppresses both head formation and budding, a reduction of the lithium concentration during a long-term treatment from 4 to 1 mM LiCl strongly promotes the differentiation of ectopic head structures. Meanwhile, budding, often interpreted as a special case of head formation, remains suppressed. The appearance of ectopic tentacles under these conditions is surprising as they develop while the animals remain in the presence of 1 mM LiCl, a concentration which would normally suppress the formation of tentacles. On a molecular level, the induction of ectopic head structures by the shift in the LiCl concentration is preceded by an increase in the level of inositol phosphates, indicating an activation of the phosphatidylinositol (PI) cycle. Increased inositol phosphate levels persist for at least 24 hr. Our results provide evidence that (i) tentacle- and bud-forming systems need different and possibly mutually exclusive physiological preconditions and (ii) prolonged inhibition of the PI cycle after initial activation might be necessary to obtain ectopic tentacles.