General equations describing the relationship between tension and length of tissue have to date been based on data obtained primarily by simple elongation, i.e. positive strains. The portion of the curve representing negative strains should be described by the same equations. In practice this portion can be found either by one-dimensional compression, or more conveniently, by uniform two-dimensional extension of tissue sheets. The entire curve cannot be predicted by extrapolation from the results of either elongation or compression alone, and must be completed experimentally. Data from four types of tissue are presented, and a suitable mathematical form describing the complete length-tension curve is given. The limitations of data from one-dimensional and uniform two-dimensional extension experiments with respect to the full description of the mechanical properties of tissue are also discussed.