The relative sizes of the various structures in Hydra attenuata were compared over a broad range of animal sizes to determine in detail the ability to regulate proportions during regeneration. The three components of the head, namely hypostome, tentacles, and tentacle zone from which the tentacles emerge, the body column, and the basal disc were all measured separately. Ectodermal cell number was used as the measure of size. The results showed that the basal disc proportioned exactly over a 40-fold size range, and the tentacle tissue proportioned exactly over a 20-fold size range. In contrast, the hypostome and tentacle zone proportioned allometrically . With decreasing size, the hypostome and tentacle zone became an increasing fraction of the animal at the expense of body tissue, and in the very smallest regenerates at the expense of tentacle tissue. In their current form, the reaction-diffusion models proposed for pattern regulation in hydra are not consistent with the data.