Small tissue pieces were excised from wild-type polyps of Hydra magnipapillata, allowed to regenerate, and the size-dependent characteristics of head regeneration were examined. The excised tissue piece was initially a square flat sheet. This piece gradually rounded up and, within 24 hr after excision, turned into a "spherical shell" which had a continuous ectodermal layer outside, a continuous endodermal layer inside, and an empty cavity at the center. The smallest spherical shell that could be produced had a diameter of 0.2 mm and contained 270-300 epithelial cells. A tissue piece too small to form a spherical shell always disintegrated, presumably due to lack of an osmotic barrier between the tissue and the environment. Of the spherical shells with a diameter of 0.2 mm, about half (47%) regenerated a head in about 5.5 days, while the rest remained as spherical shells or disintegrated. With an increase in diameter, the regeneration percentage increased, and regeneration time decreased. Similar size-dependent characteristics were also observed in a computer simulation study of regeneration based on the reaction-diffusion mechanism (Gierer and Meinhardt, Kybernetik 12, 30-39, 1972). These observations are consistent with the view that two independent factors are involved in determining the minimum tissue size for regeneration. One is the amount of tissue required to form a spherical shell. A tissue piece too small to form a spherical shell cannot survive and, hence, cannot regenerate. The other may be the reaction-diffusion mechanism. A pattern might not be formed below a critical limit size set by this mechanism.