Application of lithium ions leads to dramatic alterations in the positional value and subsequently to the formation of ectopic foot or head structures in Hydra vulgaris. Prolonged LiCl treatment decreases and pulse treatment increases the positional value. The decrease in the positional value is manifested in the formation of multiple ectopic feet along the body axis of intact animals or regenerates. Regeneration experiments reveal that long term application of the ion leads to transformation of prospective head into foot tissue. This transformation requires lithium pretreatment and exposure of the excised pieces to lithium for a further 5-8 hr. Pulse treatment of hydra with lithium elevates the positional value. Regenerating distal thirds differentiate ectopic head structures instead of a foot. Changes in the lithium concentration during the experiment lead to the differentiation of ectopic heads in the upper as well as ectopic feet in the lower body region. The characteristics of pattern respecification presented in this paper suggest that lithium interferes with the main pattern forming system(s) in hydra.