The static first hyperpolarizability tensor of model all-trans-polymethineimine oligomers has been computed ab initio as a function of chain length using the coupled Hartree-Fock method with the 6-31G basis, Careful extrapolations were carried out to obtain reliable Infinite polymer values per unit cell, For the most realistic structure the longitudinal component is about 4 times larger than any other and is of the. same order of magnitude as in push-pull polyenes. A plot of this component vs chain length exhibits a characteristic "dromedary back" shape. The role of bond alternation and heteroatomicity in determining this shape through their effect on the electronic delocalization and two different types of asymmetry (backbone, chain end) is examined qualitatively as well as quantitatively. It is shown that the correct chain length dependence is not reproduced by the uncoupled Hartree-Fock approximation.