The longitudinal static electronic and vibrational first hyperpolarizabilities of nonplanar trans-cisoid polymethineimine oligomers of increasing size have been evaluated at the Hartree-Fock 6-31G level and with the double harmonic approximation. It turns out that the vibrational static first hyperpolarizability per unit cell is very large and still increases linearly with chain length for oligomers containing fifteen unit cells, whereas its electronic counterpart tends to saturate. The vibrational component also contributes substantially to the dc-Pockels and Optical Rectification processes but not to the second harmonic generation phenomenon. The analysis of the vibrational first hyperpolarizability shows that the vibrational normal modes contributing the most have frequencies larger than 1000 cm-1 and involve variations of the bond length alternation along the chain backbone. The effects of electron correlation on the static electronic first hyperpolarizability are also addressed. (C) 2000 Elsevier Science B.V.