AbstractTo given values for the weights of twelve rats and the averages of some of them there are fitted different smooth functions by the method of least squares. Piecewise polynomial approximation before and after the turning point suggests via a differential equation an approximation by exponential functions. The composition of these exponential functions leads to an implicit function, which can approximately be substituted by an inverses polynomial. Contrary to the usual growth functions, here the mass of the animals is unbounded, what is adequate to ad libitum feeding supposing immortality. The numerical results are laid down in ten tables and they are provided with comments.